mvebu: clearfog: require uboot-mvebu-clearfog

The u-boot variant for the clearfog is provided by the uboot-mvebu-clearfog,
and not by the generic uboot-mvebu packae.
Make sure the clearfog variant is selected when building for it.

Signed-off-by: Josua Mayer <josua.mayer97@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>