boot/arm-trusted-firmware: support debug mode

boot/arm-trusted-firmware: support debug mode

When the trusted firmware is built with debug support (DEBUG defined),
the generated images are located in a different path compared to a
build without debug support. The non debug images are located in
generated directory build/<platform>/release/ while the debug images
are located in generated directory build/<platform>/debug/.

This change introduces the boolean option
BR2_TARGET_ARM_TRUSTED_FIRMWARE_DEBUG to define whether the release or
debug configuration is used to build trusted firmware.

Note that enabling trusted firmware debug support using
BR2_TARGET_ARM_TRUSTED_FIRMWARE_ADDITIONAL_VARIABLES="... DEBUG=1 ..."
would not work since Buildroot will try to copy the generated files
from the wrong path.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>