boot/arm-trusted-firmware: allow additional make targets

boot/arm-trusted-firmware: allow additional make targets

Allow specifying additional build targets for ATF.

This might be more useful when using a custom git repository.

For example, when using with the ATF repository from NXP QorIQ,
there is a new build target 'pbl' which is used to build the
pbl binary image. Note that in the specific case of the 'pbl'
target, additional build variables also need to be specified
through BR2_TARGET_ARM_TRUSTED_FIRMWARE_ADDITIONAL_VARIABLES.

Signed-off-by: Francois Gervais <fgervais@distech-controls.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>