Allow DEVTMPFS_MOUNT in kernel config (#1548)

It is important, that we have DEVTMPFS enabled in the kernel config. But
it  does not hurt to have DEVTMPFS_MOUNT enabled as well, and some of
Alpine's kernel configs have that by default. This commit removes the
check that forbids the option in kernel configs, so we can fork the
raspberry pi kernel package from Alpine without changing unrelated
options.