NOMMU: Fix the RomFS Kconfig to ensure at least one backing store is selected

Fix the configuration of the RomFS to make sure that at least one
backing store method is always selected.  This is done by rendering it
down to a choice item that selects between Block, MTD and both.

This also works correctly in the case that CONFIG_MTD=m: MTD cannot be
selected as a backing store unless CONFIG_ROMFS_FS is also 'm'.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>