build_macros: Add mechanism to prevent bin generation.

On certain platforms it does not make sense to generate
TF-A binary images. For example a platform could make use of serveral
memory areas, which are non-continuous and the resulting binary
therefore would suffer from the padding-bytes.
Typically these platforms use the ELF image.

This patch introduces a variable DISABLE_BIN_GENERATION, which
can be set to '1' in the platform makefile to prevent the binary
generation.

Signed-off-by: Christoph Müllner <christophm30@gmail.com>
Change-Id: I62948e88bab685bb055fe6167d9660d14e604462
diff --git a/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index 5d33954..2d41b2d 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -438,6 +438,11 @@
 		--script $(LINKERFILE) $(BUILD_DIR)/build_message.o \
 		$(OBJS) $(LDPATHS) $(LIBWRAPPER) $(LDLIBS) $(BL_LIBS)
 endif
+ifeq ($(DISABLE_BIN_GENERATION),1)
+	@${ECHO_BLANK_LINE}
+	@echo "Built $$@ successfully"
+	@${ECHO_BLANK_LINE}
+endif
 
 $(DUMP): $(ELF)
 	$${ECHO} "  OD      $$@"
@@ -451,7 +456,11 @@
 	@${ECHO_BLANK_LINE}
 
 .PHONY: bl$(1)
+ifeq ($(DISABLE_BIN_GENERATION),1)
+bl$(1): $(ELF) $(DUMP)
+else
 bl$(1): $(BIN) $(DUMP)
+endif
 
 all: bl$(1)