arm-trusted-firmware: cleanup make target handling

arm-trusted-firmware: cleanup make target handling

ARM_TRUSTED_FIRMWARE_MAKE_TARGET is expanded, but it's never assigned
so it is always empty. On the other hand the make targets are defined
in ARM_TRUSTED_FIRMWARE_MAKE_OPTS, which should contain options, not
targets.

Clean it all up by moving the targets in the proper place, replacing
the useless $(ARM_TRUSTED_FIRMWARE_MAKE_TARGET).

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>