WORK IN PROGRESS: Update Makefiles to get proper dependency checking working.

Fixes ARM-software/tf-issues#11

Signed-off-by: Jon Medhurst <tixy@linaro.org>
Makefile: Enable platforms to omit bootloaders

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

This also removes support for PLAT=all

Fixes ARM-software/tf-issues#30

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