* auto-created meta volume
Automatically create volume '.meta' sized 0.01% of the total space
managed by uvol, the minimum size being 4 MiB.
This volume should be used to keep metadata about stored volumes, such
as container runtime configuration (in /var/run/uvol/.meta/uxc) or
the database of installed container packages.

 * configurable LVM volume group
Allow setting volume group to be used by uvol using UCI.
This is useful on devices which do not boot off the device which
should be used for uvol and hence cannot make use of autopart and
physical volume detection based on kernel cmdline.
Use option vg_name in uvol section of fstab UCI config package.

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

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>