build_macros: Add mechanism to prevent bin generation.

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