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
diff --git a/make_helpers/plat_helpers.mk b/make_helpers/plat_helpers.mk
index a7ae9a2..bc02a20 100644
--- a/make_helpers/plat_helpers.mk
+++ b/make_helpers/plat_helpers.mk
@@ -11,6 +11,7 @@
 ifndef PLAT_HELPERS_MK
     PLAT_HELPERS_MK := $(lastword $(MAKEFILE_LIST))
 
+    PLAT:= ${DEFAULT_PLAT}
     ifeq (${PLAT},)
         $(error "Error: Unknown platform. Please use PLAT=<platform name> to specify the platform")
     endif
@@ -18,15 +19,18 @@
     # TF_PLATFORM_ROOT can be overridden for when building tools directly
     TF_PLATFORM_ROOT               ?= plat/
     PLAT_MAKEFILE               := platform.mk
+    PLAT_DEFAULTS_MAKEFILE      := platform_defaults.mk
 
     # Generate the platforms list by recursively searching for all directories
     # under /plat containing a PLAT_MAKEFILE. Append each platform with a `|`
     # char and strip out the final '|'.
     ALL_PLATFORM_MK_FILES       := $(call rwildcard,${TF_PLATFORM_ROOT},${PLAT_MAKEFILE})
+    ALL_PLATFORM_MK_DEF_FILES   := $(call rwildcard,${TF_PLATFORM_ROOT},${PLAT_DEFAULTS_MAKEFILE})
     ALL_PLATFORM_DIRS           := $(patsubst %/,%,$(dir ${ALL_PLATFORM_MK_FILES}))
     ALL_PLATFORMS               := $(sort $(notdir ${ALL_PLATFORM_DIRS}))
 
     PLAT_MAKEFILE_FULL          := $(filter %/${PLAT}/${PLAT_MAKEFILE},${ALL_PLATFORM_MK_FILES})
+    PLAT_DEFAULTS_MAKEFILE_FULL := $(filter %/${PLAT}/${PLAT_DEFAULTS_MAKEFILE},${ALL_PLATFORM_MK_DEF_FILES})
     PLATFORM_LIST               := $(subst ${space},|,${ALL_PLATFORMS})
     ifeq ($(PLAT_MAKEFILE_FULL),)
         $(error "Error: Invalid platform. The following platforms are available: ${PLATFORM_LIST}")