Introduce convenience functions to build

This patch introduces two convenience functions to the build system:

  - assert_boolean: asserts that a given option is assigned either 0 or
    1 as values

  - add_define: helps add/append macro definitions to build tool command
    line. This also introduces the variable DEFINES which is used to
    collect and pass all relevant configurations to build tools

Change-Id: I3126894b034470d39858ebb3bd183bda681c7126

Makefile

@@ -83,6 +83,19 @@ PLATFORMS		:=	$(shell ls -I common plat/)
 SPDS			:=	$(shell ls -I none services/spd)
 HELP_PLATFORMS		:=	$(shell echo ${PLATFORMS} | sed 's/ /|/g')
 
+# Convenience function for adding build definitions
+# $(eval $(call add_define,FOO)) will have:
+# -DFOO if $(FOO) is empty; -DFOO=$(FOO) otherwise
+define add_define
+DEFINES			+=	-D$(1)$(if $(value $(1)),=$(value $(1)),)
+endef
+
+# Convenience function for verifying option has a boolean value
+# $(eval $(call assert_boolean,FOO)) will assert FOO is 0 or 1
+define assert_boolean
+$(and $(patsubst 0,,$(value $(1))),$(patsubst 1,,$(value $(1))),$(error $(1) must be boolean))
+endef
+
 ifeq (${PLAT},)
   $(error "Error: Unknown platform. Please use PLAT=<platform name> to specify the platform.")
 endif
@@ -131,7 +144,6 @@ endif
 .PHONY:			all msg_start clean realclean distclean cscope locate-checkpatch checkcodebase checkpatch fiptool fip
 .SUFFIXES:
 
-
 INCLUDES		+=	-Iinclude/bl1			\
 				-Iinclude/bl2			\
 				-Iinclude/bl31			\