build: Extract trusted-firmware-a.mk

build: Extract trusted-firmware-a.mk

The include/trusted-firmware-a.mk file is based on the
include/u-boot.mk file and should be used to build a Trusted Firmware-A
(TFA) which was previously named Arm trusted firmware.

This is useful for targets where the TFA is board specific like for
Marvell SoCs and probably also NXP Layerscape SoCs.

This also makes use of this abstraction in the
arm-trusted-firmware-mvebu package to build board specific ATF binaries.

The ATF binaries will be automatically activated and build when the
board is selected in the normal build or all boards are selected. This
should also activate the build when build bot creates images.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>