u-boot makefile问题

[wzw88486969]

$(obj)u-boot: depend $(SUBDIRS) $(OBJS) $(LIBBOARD) $(LIBS) $(LDSCRIPT)
UNDEF_SYM=`$(OBJDUMP) -x $(LIBBOARD) $(LIBS) | \
sed -n -e 's/.*\($(SYM_PREFIX)__u_boot_cmd_.*\)/-u\1/p'|sort|uniq`;\
cd $(LNDIR) && $(LD) $(LDFLAGS) $$UNDEF_SYM $(__OBJS) \
--start-group $(__LIBS) --end-group $(PLATFORM_LIBS) \
-Map u-boot.map -o u-boot

$(OBJS): depend $(obj)include/autoconf.mk
$(MAKE) -C cpu/$(CPU) $(if $(REMOTE_BUILD),$@,$(notdir $@))

$(LIBS): depend $(obj)include/autoconf.mk
$(MAKE) -C $(dir $(subst $(obj),,$@))


depend dep: $(VERSION_FILE)
for dir in $(SUBDIRS) ; do $(MAKE) -C $$dir _depend ; done

想问问各位，depend这个是什么用啊 后面的怎么每个都依赖他啊 ，
还有 $(MAKE) -C $(dir $(subst $(obj),,$@))是怎么循环进入每个子目录啊，
这个是u-boot的makefile 那位帮帮说说吧 谢谢 谢谢！！