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>
diff --git a/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index 5de810b..bf9dc79 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -184,7 +184,7 @@
 # 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