build: Use filechk rules to create and update u-boot.lds

Prior to this patch, the top-level linker script u-boot.lds
used a simple $(call if_changed) check when generated.
That mechanism misses cases where a possible include file
change induces a change in the u-boot.lds too.

This patch converts it to a stronger check using ($call filechk)
that will also notice differences in file contents and
will catch changes due to pre-processing as well.

Signed-off-by: Jon Loeliger <jon.loeliger@oracle.com>

Makefile

@@ -1093,12 +1093,13 @@ depend dep:
 	@echo '*** Warning: make $@ is unnecessary now.'
 
 # ---------------------------------------------------------------------------
-quiet_cmd_cpp_lds = LDS     $@
-cmd_cpp_lds = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
-		-D__ASSEMBLY__ -x assembler-with-cpp -P -o $@ $<
+define filechk_ubootlds
+	($(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
+		-D__ASSEMBLY__ -x assembler-with-cpp -P -o - -)
+endef
 
 u-boot.lds: $(LDSCRIPT) prepare FORCE
-	$(call if_changed_dep,cpp_lds)
+	$(call filechk,ubootlds)
 
 PHONY += nand_spl
 nand_spl: prepare