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>