build system: allow overriding the build's timestamp

This allows reproducible builds (same source and same compiler produce
bit-identical results) and also allows coordinating the timestamp across
multiple projects, eg. with another firmware.
Signed-off-by: Patrick Georgi <pgeorgi@google.com>

make_helpers/build_macros.mk

@@ -290,6 +290,10 @@ define MAKE_TOOL_ARGS
         $(if $(3),$(eval $(call FIP_ADD_PAYLOAD,$(2),--$(3),bl$(1))))
 endef
 
+# Allow overriding the timestamp, for example for reproducible builds, or to
+# synchronize timestamps across multiple projects.
+# This must be set to a C string (including quotes where applicable).
+BUILD_MESSAGE_TIMESTAMP ?= __TIME__", "__DATE__
 
 # MAKE_BL macro defines the targets and options to build each BL image.
 # Arguments:
@@ -315,7 +319,7 @@ $(BUILD_DIR):
 
 $(ELF): $(OBJS) $(LINKERFILE)
 	@echo "  LD      $$@"
-	@echo 'const char build_message[] = "Built : "__TIME__", "__DATE__; \
+	@echo 'const char build_message[] = "Built : "$(BUILD_MESSAGE_TIMESTAMP); \
 	       const char version_string[] = "${VERSION_STRING}";' | \
 		$$(CC) $$(CFLAGS) -xc - -o $(BUILD_DIR)/build_message.o
 	$$(Q)$$(LD) -o $$@ $$(LDFLAGS) -Map=$(MAPFILE) --script $(LINKERFILE) \