As /var/state is used for UCI state, move persistent config for uxc
from /var/state/uxc to /var/run/uxc.
uxc itself has already been changed to look for it there.

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>

* genrate UCI fstab configs for each volume before first 'up'
 * remove UCI section on volume remove
 * use autofs automounter for read-only volumes
 * try umount on 'down'
 * emulate hotplug events for UBI volume up/down
 * more robust error paths

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