Set the IMAGE_BLx flag for the linker preprocessor

Change-Id: Ibc91f119c99413ded59a9db3db918d22f0517bc1
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>

make_helpers/build_macros.mk

@@ -252,10 +252,11 @@ endef
 define MAKE_LD
 
 $(eval DEP := $(1).d)
+$(eval IMAGE := IMAGE_BL$(call uppercase,$(3)))
 
 $(1): $(2) $(filter-out %.d,$(MAKEFILE_LIST)) | bl$(3)_dirs
 	@echo "  PP      $$<"
-	$$(Q)$$(CPP) $$(CPPFLAGS) -P -D__ASSEMBLY__ -D__LINKER__ $(MAKE_DEP) -o $$@ $$<
+	$$(Q)$$(CPP) $$(CPPFLAGS) -P -D__ASSEMBLY__ -D__LINKER__ $(MAKE_DEP) -D$(IMAGE) -o $$@ $$<
 
 -include $(DEP)