core: link.mk: force link of tee.elf when object list has changed

Since the parent commit ("Add support for early Trusted Applications"),
the link step of tee.elf may pull object files generated from Trusted
Application binaries. This is controlled by $(EARLY_TA_PATHS). Adding
or removing files should cause tee.elf to be re-linked, even when no
re-compilation or change in the configuration variables occur. This is
not the case currently. For example:
 $ make EARLY_TA_PATHS="a.elf b.elf"
 $ make EARLY_TA_PATHS="a.elf"  # Should re-link without b.elf

The link recipe is modified so that the link step is forced when the
object list changes.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/kernel/link.mk

@@ -131,12 +131,20 @@ endef
 $(link-out-dir)/version.o:
 	$(call gen-version-o)
 
+-include $(link-out-dir)/.tee.elf.cmd
+define check-link-objs
+$(if $(strip $(filter-out $(link-objs), $(old-link-objs))
+	     $(filter-out $(old-link-objs), $(link-objs))), FORCE_LINK := FORCE)
+endef
+$(eval $(call check-link-objs))
 
 all: $(link-out-dir)/tee.elf
 cleanfiles += $(link-out-dir)/tee.elf $(link-out-dir)/tee.map
 cleanfiles += $(link-out-dir)/version.o
 cleanfiles += $(link-out-dir)/.buildcount
-$(link-out-dir)/tee.elf: $(link-objs) $(libdeps) $(link-script-pp)
+cleanfiles += $(link-out-dir)/.tee.elf.cmd
+$(link-out-dir)/tee.elf: $(link-objs) $(libdeps) $(link-script-pp) $(FORCE_LINK)
+	@echo "old-link-objs := $(link-objs)" >$(link-out-dir)/.tee.elf.cmd
 	@$(cmd-echo-silent) '  LD      $@'
 	$(q)$(LDcore) $(ldargs-tee.elf) -o $@