Enable platforms to omit some bootloaders

If a platform doesn't specify a BLx_SOURCE variable, then building
of the corresponding bootloader isn't attempted.

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

Fixes ARM-software/tf-issues#30

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