build_macros: Add mechanism to prevent bin generation.

On certain platforms it does not make sense to generate
TF-A binary images. For example a platform could make use of serveral
memory areas, which are non-continuous and the resulting binary
therefore would suffer from the padding-bytes.
Typically these platforms use the ELF image.

This patch introduces a variable DISABLE_BIN_GENERATION, which
can be set to '1' in the platform makefile to prevent the binary
generation.

Signed-off-by: Christoph Müllner <christophm30@gmail.com>
Change-Id: I62948e88bab685bb055fe6167d9660d14e604462
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index be84f77..dc797ed 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -62,6 +62,9 @@
 # Build platform
 DEFAULT_PLAT			:= fvp
 
+# Disable the generation of the binary image (ELF only).
+DISABLE_BIN_GENERATION		:= 0
+
 # Enable capability to disable authentication dynamically. Only meant for
 # development platforms.
 DYN_DISABLE_AUTH		:= 0