trusted-firmware-a: support multi-board platforms

Some platforms can have multiple board configurations, passed as TARGET_BOARD=""
that also becomes an extra directory level in the build output hierarchy.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>