Make:Use environment variables for OS detection.

Add make helper files to select the appropriate settings for the build
environment. Selection is made in make_helpers/build_env.mk, which
selects other files to include using generic build environment settings.
The Trusted Firmware Makefile and supporting tool Makefiles are updated
to include build_env.mk instead of unix.mk.

NOTE: This change does not fully enable builds in other build
      environments. It facilitates this without compromising the
      existing build environments.

Change-Id: Ic4064ffe6ce158bbd16d7cc9f27dd4655a3580f6
diff --git a/tools/cert_create/Makefile b/tools/cert_create/Makefile
index b19d618..8218ab7 100644
--- a/tools/cert_create/Makefile
+++ b/tools/cert_create/Makefile
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2016, ARM Limited and Contributors. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are met:
@@ -49,7 +49,7 @@
 
 MAKE_HELPERS_DIRECTORY := ../../make_helpers/
 include ${MAKE_HELPERS_DIRECTORY}build_macros.mk
-include ${MAKE_HELPERS_DIRECTORY}unix.mk
+include ${MAKE_HELPERS_DIRECTORY}build_env.mk
 
 PLATFORM_ROOT		:=	../../plat/
 include ${MAKE_HELPERS_DIRECTORY}plat_helpers.mk
diff --git a/tools/fip_create/Makefile b/tools/fip_create/Makefile
index 9f0bf7a..52e06a7 100644
--- a/tools/fip_create/Makefile
+++ b/tools/fip_create/Makefile
@@ -30,7 +30,7 @@
 
 MAKE_HELPERS_DIRECTORY := ../../make_helpers/
 include ${MAKE_HELPERS_DIRECTORY}build_macros.mk
-include ${MAKE_HELPERS_DIRECTORY}unix.mk
+include ${MAKE_HELPERS_DIRECTORY}build_env.mk
 
 PROJECT = fip_create
 OBJECTS = fip_create.o