uboot-mvebu: make hidden and be m for clearfog to fix IB failing to add it

Uboot-mvebu isn't a real package, which will break the image builder
when it tries to install it during the packing step. Instead of cleafog
selecting it through its default packages, make it default to m if the
clearfog profile is selected.

This will ensure it is always build, but never added to the rootfs. This
fixes creating images for clearfog with IB.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Felix Fietkau <nbd@nbd.name>