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If a platform doesn't specify a BLx_SOURCE variable, then building
of the corresponding bootloader isn't attempted. Also allow BL3-3 to
be omitted from the FIP.

Note, this change also removes support for PLAT=all and the 'fip' target
from the 'all' recipe.

Fixes ARM-software/tf-issues#30

Change-Id: Ibdfead0440256eaf364617ecff65290ca6fe6240
Signed-off-by: Jon Medhurst <tixy@linaro.org>