feat(build): add ability to define platform specific defaults

In some cases it maybe needed to override some default settings on a
particular platform. For example, enable ENABLE_LTO on a size
constrained platform.

Change-Id: I556d26f6b81c0f3ceb40b7196180995dde22afd0
Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
diff --git a/Makefile b/Makefile
index e157022..a2ff9ce 100644
--- a/Makefile
+++ b/Makefile
@@ -32,6 +32,12 @@
 ################################################################################
 
 include ${MAKE_HELPERS_DIRECTORY}defaults.mk
+include ${MAKE_HELPERS_DIRECTORY}plat_helpers.mk
+
+# To be able to set platform specific defaults
+ifneq ($(PLAT_DEFAULTS_MAKEFILE_FULL),)
+include ${PLAT_DEFAULTS_MAKEFILE_FULL}
+endif
 
 ################################################################################
 # Configure the toolchains used to build TF-A and its tools
@@ -53,7 +59,6 @@
 # Assertions enabled for DEBUG builds by default
 ENABLE_ASSERTIONS		:= ${DEBUG}
 ENABLE_PMF			:= ${ENABLE_RUNTIME_INSTRUMENTATION}
-PLAT				:= ${DEFAULT_PLAT}
 
 ################################################################################
 # Checkpatch script options
@@ -418,7 +423,6 @@
 ################################################################################
 # Generic definitions
 ################################################################################
-include ${MAKE_HELPERS_DIRECTORY}plat_helpers.mk
 
 ifeq (${BUILD_BASE},)
      BUILD_BASE		:=	./build