ARM: mvebu: only build kwbootimage support if BOOTM is enabled
As this is the bootm image handler implementation for the kwbootimage
it is of no use if BOOTM is missing and fails to link.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>