Add support for specifying pre-built BL binaries in Makefile

This patch adds support for supplying pre-built BL binaries for BL2,
BL31 and BL32 during trusted firmware build. Specifying BLx = <path_to_BLx>
in the build command line, where 'x' is any one of BL2, BL31 or BL32, will
skip building that BL stage from source and include the specified binary in
final fip image.

This patch also makes BL33 binary for FIP optional depending on the
value of 'NEED_BL33' flag which is defined by the platform.

Fixes ARM-software/tf-issues#244
Fixes ARM-software/tf-issues#245

Change-Id: I3ebe1d4901f8b857e8bb51372290978a3323bfe7