Emmit ubus events when volumes come up/down.
Make sure volume state is always well defined by introducing
additional state 'write-prepare' (wp) during mkfs.
Add init scripts to bring up volumes at boot.

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

uvol is a wrapper-script which allows automated handling of storage
volumes. uvol currently comes with backend support for LVM2 and UBI,
covering practically all options for storage large enough to be
managed (NAND, SPI-NAND, eMMC, SATA, NVME, virtio-blk, ...).

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