Create UCI fstab mount sections for existing volumes on firstboot.
Introduce special handling for volumes with non-[:alnum:] prefix:
 - do not create UCI sections for these 'hidden' volumes
 - create UCI section for '.uxc' special volume with target
   /var/state/uxc to be used for uxc config.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>