arm-bsp: linux-linaro-arm: Set COMPATIBLE_MACHINE to "invalid"

To keep consistency with trusted-firmware-a and optee recipes we should
use softer assignment to "invalid" for COMPATIBLE_MACHINE.

Change-Id: I57d0b220d92c4546e0539f95a54d391935f995fc
Issue-Id: SCM-954
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>