Makefile: Allow arch post-link hook

This commit allows an architecture to provide a Makefile.postlink whose
u-boot target gets invoked after the u-boot ELF is linked. This will be
of use for MIPS in a following commit.

This mirrors Linux commit fbe6e37dab97 ("kbuild: add arch specific
post-link Makefile").

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: u-boot@lists.denx.de
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Tested-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

Makefile

@@ -1232,13 +1232,16 @@ u-boot.elf: u-boot.bin
 	$(Q)$(OBJCOPY) -I binary $(PLATFORM_ELFFLAGS) $< u-boot-elf.o
 	$(call if_changed,u-boot-elf)
 
+ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(ARCH)/Makefile.postlink)
+
 # Rule to link u-boot
 # May be overridden by arch/$(ARCH)/config.mk
 quiet_cmd_u-boot__ ?= LD      $@
       cmd_u-boot__ ?= $(LD) $(LDFLAGS) $(LDFLAGS_u-boot) -o $@ \
       -T u-boot.lds $(u-boot-init)                             \
       --start-group $(u-boot-main) --end-group                 \
-      $(PLATFORM_LIBS) -Map u-boot.map
+      $(PLATFORM_LIBS) -Map u-boot.map;                        \
+      $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
 quiet_cmd_smap = GEN     common/system_map.o
 cmd_smap = \
@@ -1248,7 +1251,7 @@ cmd_smap = \
 		-c $(srctree)/common/system_map.c -o common/system_map.o
 
 u-boot:	$(u-boot-init) $(u-boot-main) u-boot.lds FORCE
-	$(call if_changed,u-boot__)
+	+$(call if_changed,u-boot__)
 ifeq ($(CONFIG_KALLSYMS),y)
 	$(call cmd,smap)
 	$(call cmd,u-boot__) common/system_map.o