在编译ZedBoard的u-boot.elf（ssbl）中知道了编译u-boot的步骤，以及u-boot下载地址，这里使用2014.4版本。由于现在使用的是zc702开发板，所以应该使用如下命令编译u-boot：
1、make zynq_zc70x_config
2、make
其中第1条命令和第2条命令都会执行根目录下的Makefile文件，但是在Makefile文件中会判断make命令后是否跟了带_config的参数。如果带了，就会执行mkconfig -A zynq_zc70x这条命令。否则就执行生成u-boot.bin的那一大段命令。
这里mkconfig做了几件事：
1、从boards.cfg中获取目标板参数，并将参数传给8个变量。
2、./arch/arm/include/asm下进行一些文件夹链接。
3、在./include/目录下生成config.mk和config.h文件。
#!/bin/sh -e
# Script to createheader files and links to configure
# U-Boot for aspecific board.
#
# Parameters: Target Architecture CPU Board [VENDOR] [SOC]
#
# (C) 2002-2013DENX Software Engineering, Wolfgang Denk <[email protected]>
#
#SPDX-License-Identifier: GPL-2.0+
#
APPEND=no # Default: Create new config file
BOARD_NAME="" # Name to print in make output
TARGETS=""
arch=""
cpu=""
board=""
vendor=""
soc=""
options=""
#我们执行的命令是mkconfig –A zynq_zc70x，所以$#值为2，$1值为-A，以下条件满足
if [ \( $# -eq 2\) -a \( "$1" = "-A" \) ] ; then
# Automatic mode
line=`awk ‘($0 !~ /^#/ && $7 ~/^‘"$2"‘$/) { print $1, $2, $3, $4, $5, $6, $7, $8 }‘$srctree/boards.cfg` #在board.cfg中找到zynq_zc70x对应的那行，并将整行值付给line。
if [ -z "$line" ] ; then #line变量为null
表示没有在boards.cfg中找到目标板信息
echo "make: *** No rule tomake target \`$2_config‘. Stop.">&2
exit 1
fi
set ${line} #set用于在脚本内部给出其运行参数，所以这个时候脚本运行参数就变为line的值：Active arm armv7 zynq xilinx zynq zynq_zc70x -
# add default board name if needed
[ $# = 3 ] && set ${line} ${1}
fi
#由于我们并没有传递给mkconfig任何的选项，因此while循环中的代码不起作用。
while [ $# -gt0 ] ; do
case "$1" in
--) shift ; break ;;
-a) shift ; APPEND=yes ;;
-n) shift ;BOARD_NAME="${7%_config}" ; shift ;;
-t) shift ; TARGETS="`echo $1 | sed‘s:_: :g‘` ${TARGETS}" ; shift ;;
*) break ;;
esac
done
#参数个数小于7或者大于8退出，现在参数为8，不退出
[ $# -lt 7 ]&& exit 1
[ $# -gt 8 ]&& exit 1
#${7%_config}的意思是 如果$7以 _config 后缀结束的话，则要去掉 _config后缀。由于现在$7=zynq_zc70x，所以CONFIG_NAME就等zynq_zc70x
# Strip alloptions and/or _config suffixes
CONFIG_NAME="${7%_config}"
[ "${BOARD_NAME}"] || BOARD_NAME="${7%_config}" #BOARD_NAME为空，所以此时BOARD_NAME也等于zynq_zc70x。
arch="$2" #arch=arm
cpu=`echo $3 | awk‘BEGIN {FS = ":"} ; {print $1}‘` #cpu=armv7
spl_cpu=`echo $3 |awk ‘BEGIN {FS = ":"} ; {print $2}‘`
#spl_cpu=armv7
if ["$cpu" = "-" ] ; then
cpu=
fi
[ "$6"!= "-" ] && board="$6" #board=zynq
[ "$5"!= "-" ] && vendor="$5" #vendor=xilinx
[ "$4"!= "-" ] && soc="$4" #soc=zynq
#$8为”-“，不执行代码
[ $# -gt 7 ]&& [ "$8" != "-" ] && {
# check if we have a board config name inthe options field
# the options field mave have a boardconfig name and a list
# of options, both separated by a colon(‘:‘); the options are
# separated by commas (‘,‘).
#
# Check for board name
tmp="${8%:*}"
if [ "$tmp" ] ; then
CONFIG_NAME="$tmp"
fi
# Check if we only have a colon...
if [ "${tmp}" != "$8"] ; then
options=${8#*:}
TARGETS="`echo ${options} |sed ‘s:,: :g‘` ${TARGETS}"
fi
}
#如果ARCH已经有值了，那么就检测ARCH和arch是否匹配了。（现在ARCH就为arch）
if ["${ARCH}" -a "${ARCH}" != "${arch}" ]; then
echo "Failed: \$ARCH=${ARCH}, shouldbe ‘${arch}‘ for ${BOARD_NAME}" 1>&2
exit 1
fi
#
# Test aboveneeded aarch64, now we need arm
#
if ["${arch}" = "aarch64" ]; then
arch="arm"
fi
#此时options为空，所以输出Configuringfor zynq_zc70x board…
if ["$options" ] ; then
echo "Configuring for ${BOARD_NAME}- Board: ${CONFIG_NAME}, Options: ${options}"
else
echo "Configuring for ${BOARD_NAME}board..."
fi
#以下操作主要是创建一些文件夹链接
#KBUILD_SRC、objtree、srctree的路径值都是在Makefile中执行mkconfig -A zynq_zc70x命令之前设置好的。
#KBUILD_SRC为空，objtree、srctree为根目录路径
#
# Create link toarchitecture specific headers
#
if [ -n "$KBUILD_SRC" ] ; then #KBUILD_SRC为空，所以进入else语句
mkdir -p ${objtree}/include
LNPREFIX=${srctree}/arch/${arch}/include/asm/
cd ${objtree}/include
mkdir -p asm
else
cd arch/${arch}/include #进入./arch/arm/include目录
fi
rm -f asm/arch #直接删除当前目录下asm/arch目录
if ["${soc}" ] ; then #soc为zynq，进入if分支
ln -s ${LNPREFIX}arch-${soc} asm/arch #将asm/arch链接为arch-zynq目录，LNPREFIX=””
elif ["${cpu}" ] ; then
ln -s ${LNPREFIX}arch-${cpu} asm/arch
fi
if [ -z"$KBUILD_SRC" ] ; then #KBUILD_SRC为空，进入根目录下include目录
cd ${srctree}/include
fi
#
# Create includefile for Make
###################################################
执行如下代码后，在根目录include目录下创建config.mk文件并写入：
ARCH = arm
CPU = armv7
BOARD = zynq
VENDOR =xilinx
SOC = zynq
###################################################
( echo"ARCH = ${arch}"
if [ ! -z "$spl_cpu" ] ; then
echo ‘ifeq ($(CONFIG_SPL_BUILD),y)‘
echo "CPU = ${spl_cpu}"
echo "else"
echo "CPU = ${cpu}"
echo "endif"
else
echo "CPU = ${cpu}"
fi
echo "BOARD = ${board}"
[ "${vendor}" ] && echo"VENDOR = ${vendor}"
[ "${soc}" ] && echo "SOC = ${soc}"
exit 0 ) >config.mk
# Assign boarddirectory to BOARDIR variable
if [ -z"${vendor}" ] ; then #vendor不为空，进入else分支
BOARDDIR=${board}
else
BOARDDIR=${vendor}/${board} #BOARDDIR=xilinx/zynq
fi
#
# Create boardspecific header file
#####################################################
执行以下代码后，在根目录include目录下创建config.h文件并写入：
/*Automatically generated - do not edit */
#defineCONFIG_SYS_ARCH "arm"
#define CONFIG_SYS_CPU "armv7"
#defineCONFIG_SYS_BOARD "zynq"
#defineCONFIG_SYS_VENDOR "xilinx"
#defineCONFIG_SYS_SOC "zynq"
#defineCONFIG_BOARDDIR board/xilinx/zynq
#include<config_cmd_defaults.h>
#include<config_defaults.h>
#include<configs/zynq_zc70x.h>
#include<asm/config.h>
#include<config_fallbacks.h>
#include<config_uncmd_spl.h>
#####################################################
if ["$APPEND" = "yes" ] #Append to existing config file
then
echo >> config.h
else
> config.h # Create new config file
fi
echo "/*Automatically generated - do not edit */" >>config.h
for i in${TARGETS} ; do
i="`echo ${i} | sed ‘/=/ {s/=/ /;q; } ; { s/$/ 1/; }‘`"
echo "#define CONFIG_${i}">>config.h ;
done
echo "#defineCONFIG_SYS_ARCH \"${arch}\"" >> config.h
echo "#defineCONFIG_SYS_CPU \"${cpu}\"" >> config.h
echo "#defineCONFIG_SYS_BOARD \"${board}\"" >> config.h
["${vendor}" ] && echo "#define CONFIG_SYS_VENDOR\"${vendor}\"" >> config.h
["${soc}" ] && echo"#define CONFIG_SYS_SOC \"${soc}\"" >> config.h
["${board}" ] && echo"#define CONFIG_BOARDDIR board/$BOARDDIR" >> config.h
#将输入的内容追加到config.h中，直到出现“EOF”这样的标识为止。
cat << EOF>> config.h
#include<config_cmd_defaults.h>
#include<config_defaults.h>
#include<configs/${CONFIG_NAME}.h>
#include<asm/config.h>
#include<config_fallbacks.h>
#include<config_uncmd_spl.h>
EOF
exit 0