west: sign: add support for NXP i.MX8QXP board Add support for signing i.MX8QXP SOF with Zephyr images with rimage. i.MX8QXP and i.MX8QM have the same board-level definitions, so we use the generic imx8. Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>