tbbr-tools: enable override TRUSTED_KEY_CERT
tbbr-tools: enable override TRUSTED_KEY_CERT

Platforms, which requires additional images to be
verified using TBBR; such that their key certificate
is tied to TRUSTED_KEY_CERT.

For such platforms, if make commands runs twice:
 - Once with targets as bl2 & fip.bin, and
 - Again to build the target as the additional image.

then, if path to the TRUSTED_KEY_CERT varies in the
makefile with make-target of the additional image, then
there would be two location where "trusted_key.crt" will
be created.

This patch helps overriding the TRUSTED_KEY_CERT from any .mk
in the platform's makefile structure.

Signed-off-by: Pankaj Gupta <c3ec4fc81871614951009ffb2adac92410a42e25@nxp.com>
Change-Id: I775a2c409035504b21b0bbe5a4f9046898163eed

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware