编译u-boot的步骤：
#make XX_config XX表示某个cpu体系
#make 生成我们需要的u-boot.bin
具体可参考u-boot文件中的README。
1.设置版本
VERSION = 2010
PATCHLEVEL = 06
SUBLEVEL =
EXTRAVERSION = -rc1
ifneq "$(SUBLEVEL)" ""
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
else
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL)$(EXTRAVERSION)
endif
TIMESTAMP_FILE = $(obj)include/timestamp_autogenerated.h
VERSION_FILE = $(obj)include/version_autogenerated.h
以上定义了版本的变量U_BOOT_VERSION。
2.获取主机类型和主机系统
HOSTARCH := $(shell uname -m | \ 执行shell命令 uname -m 查看主机类型
sed -e s/i.86/i386/ \
-e s/sun4u/sparc64/ \
-e s/arm.*/arm/ \
-e s/sa110/arm/ \
-e s/ppc64/powerpc/ \
-e s/ppc/powerpc/ \
-e s/macppc/powerpc/)
sed -e命令进行替换，比如将i386替换成i.86，并将结果放入变量HOSTARCH 。
HOSTOS := $(shell uname -s | tr '[:upper:]' '[:lower:]' | \
sed -e 's/\(cygwin\).*/cygwin/')
uname -s 查看主机操作系统，tr '[:upper:]' '[:lower:]'将所有大写变小写，然后假如有cygwin，替换成cygwin.*，并将结果放入变量HOSTOS
# Set shell to bash if possible, otherwise fall back to sh
SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
else if [ -x /bin/bash ]; then echo /bin/bash; \
else echo sh; fi; fi)
选择/bin/bash
export HOSTARCH HOSTOS SHELL
导出变量HOSTARCH HOSTOS SHELL
LC_ALL=C
export LC_ALL
# Deal with colliding definitions from tcsh etc.
VENDOR=
变量为空
#########################################################################
# Allow for silent builds
ifeq (,$(findstring s,$(MAKEFLAGS)))
XECHO = echo
else
XECHO = :
endif
#########################################################################
#
# U-boot build supports producing a object files to the separate external
# directory. Two use cases are supported:
#
# 1) Add O= to the make command line
# 'make O=/tmp/build all'
#
# 2) Set environement variable BUILD_DIR to point to the desired location
# 'export BUILD_DIR=/tmp/build'
# 'make'
#
# The second approach can also be used with a MAKEALL script
# 'export BUILD_DIR=/tmp/build'
# './MAKEALL'
#
# Command line 'O=' setting overrides BUILD_DIR environent variable.
#
# When none of the above methods is used the local build is performed and
# the object files are placed in the source directory.
#
U-boot 生成一个目标文件来分开外面的文件夹，支持两种方式：
1）将 0= 加到make命令行如：
make 0=/tmp/build all
2) 通过设置全局环境变量BUILD_DIR如：
export BUILD_DIR=/tmp/build
make
第二种方式也可以用MAKEFILE脚本
export BUILD_DIR=/tmp/build
./MAKEALL
通过第一种方式会覆盖环境变量 BUILD_DIR
如果没有显式指定使用哪种方式，编译脚本会自动进行本地编译，目标文件被存放在当前文件夹里