在uboot中，所写的shell脚本：mx6ull_alientek_emmc.sh的内容如下：

#!/bin/bash
  2 make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- distclean
  3 make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- mx6ull_14x14_ddr512_emmc_defconfig
  4 make V=1 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j12

首先是清理工程
其次就是make mx6ull_14x14_ddr512_emmc_defconfig
最后就是编译

在编译uboot之前要使用“make xxx_deconfig”命令来配置值uboot，顶层Makefile中有如下代码：

# To make sure we do not include .config for any of the *config targets
# catch them early, and hand them over to scripts/kconfig/Makefile
# It is allowed to specify more targets when calling make, including
# mixing *config targets and build targets.
# For example 'make oldconfig all'.
# Detect when mixed targets is specified, and make a second invocation
# of make so .config is not included in this case either (for *config).

version_h := include/generated/version_autogenerated.h
timestamp_h := include/generated/timestamp_autogenerated.h

no-dot-config-targets := clean clobber mrproper distclean \
			 help %docs check% coccicheck \
			 ubootversion backup

config-targets := 0
mixed-targets  := 0
dot-config     := 1

ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
	ifeq ($(filter-out $(no-dot-config-targets), $(MAKECMDGOALS)),)
		dot-config := 0
	endif
endif

ifeq ($(KBUILD_EXTMOD),)
        ifneq ($(filter config %config,$(MAKECMDGOALS)),)
                config-targets := 1
                ifneq ($(words $(MAKECMDGOALS)),1)
                        mixed-targets := 1
                endif
        endif
endif

ifeq ($(mixed-targets),1)
# ===========================================================================
# We're called with mixed targets (*config and build targets).
# Handle them one by one.

PHONY += $(MAKECMDGOALS) __build_one_by_one

$(filter-out __build_one_by_one, $(MAKECMDGOALS)): __build_one_by_one
	@:

__build_one_by_one:
	$(Q)set -e; \
	for i in $(MAKECMDGOALS); do \
		$(MAKE) -f $(srctree)/Makefile $$i; \
	done

else
ifeq ($(config-targets),1)
# ===========================================================================
# *config targets only - make sure prerequisites are updated, and descend
# in scripts/kconfig to make the *config target

KBUILD_DEFCONFIG := sandbox_defconfig
export KBUILD_DEFCONFIG KBUILD_KCONFIG

config: scripts_basic outputmakefile FORCE
	$(Q)$(MAKE) $(build)=scripts/kconfig $@

%config: scripts_basic outputmakefile FORCE
	$(Q)$(MAKE) $(build)=scripts/kconfig $@

else
# ===========================================================================
# Build targets only - this includes vmlinux, arch specific targets, clean
# targets and others. In general all targets except *config targets.

ifeq ($(dot-config),1)
# Read in config
-include include/config/auto.conf

• 定义了变量version_h，此变量保存版本号文件，此文件是自动生成的。
• timestamp_h保存时间戳文件，此文件也是自动生成的。
• 将MAKECMDGOALS中不符合no-dot-config-targets的部分过滤掉，剩下的如果不为空的话就条件成立。MAKECMDGOALS是make的一个环境变量，会保存指定的终极目标列表，比如执行“make mx6ull_alientek_emmc_defconfig”，那么 MAKECMDGOALS就为 mx6ull_alientek_emmc_defconfig。很明显过滤后为空，所以条件不成立，变量dot-config依旧为1。

config: scripts_basic outputmakefile FORCE
	$(Q)$(MAKE) $(build)=scripts/kconfig $@

%config: scripts_basic outputmakefile FORCE
	$(Q)$(MAKE) $(build)=scripts/kconfig $@

有目标与之匹配，当输入“make xxx_defconfig”的时候就会匹配到%config目标，目标“%config”依赖于scripts_basic、outputmakefile、FORCE。

FORCE在顶层Makefile的1610行有如下定义

PHONY += FORCE
FORCE:

可以看出FORCE是没有规则和依赖的，所以每次都会重新生成FORCE。
当FORCE作为其它目标的依赖时，由于FORCE总是被更新过的，所以依赖所在的规则总是会执行的。
outputmakefile 无效，只有 scripts_basic 是有效的。

Q=@或为空
MAKE=make

变量build是在scripts/Kbuild.include 文件中有定义

###
# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=
# Usage:
# $(Q)$(MAKE) $(build)=dir
build := -f $(srctree)/scripts/Makefile.build obj

build =-f ./scripts/Makefile.build obj

scripts_basic展开后

scripts_basic:
@make -f ./scripts/Makefile.build obj=scripts/basic //也可以没有@，视配置而定
@rm -f . tmp_quiet_recordmcount //也可以没有@

所以最后

%config: scripts_basic outputmakefile FORCE
$(Q)$(MAKE) $(build)=scripts/kconfig $@

展开为：

@make -f ./scripts/Makefile.build obj=scripts/kconfig xxx_defconfig

配置uboot的过程

make mx6ull_14x14_ddr512_emmc_defconfig V=1

scripts/Makfile.build中
scripts_basic 目标对应的命令

@make -f ./scripts/Makefile.build obj=scripts/basic

%config 目标对应的命令

@make -f ./scripts/Makefile.build obj=scripts/kconfig xxx_defconfig