Build: add -MP option to add dummy rules to *.d files

This adds a phony target for each dependency other than the main
file, causing each to depend on nothing.

Without this, the incremental build will fail when a header file
is removed.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

make_helpers/build_macros.mk

@@ -184,7 +184,7 @@ endef
 # Auxiliary macros to build TF images from sources
 ################################################################################
 
-MAKE_DEP = -Wp,-MD,$(DEP) -MT $$@
+MAKE_DEP = -Wp,-MD,$(DEP) -MT $$@ -MP
 
 # MAKE_C builds a C source file and generates the dependency file
 #   $(1) = output directory