make: support libraries in MAKE_BL macro

Add support for BLx stages to use libraries in MAKE_BL macro.
This change does not affect BL stages that do not have
BL_LIBS variable defined in their makefiles.
However in case that BL wants to use external library
(for instance vendor-specific DDR initialization code supplied
as a library), this patch will allow to build BL image linked
with such library.

Change-Id: Ife29069a72dc4aff833db6ef8b828736d6689b78
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>