uvol: add new package

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>