fstools: add partname volume driver

fstools: add partname volume driver

Add driver to handle block devices with (GPT) partition table which
can include a partition name.
If 'root=' is set on the kernel cmdline, only search for partitions
on the same device.
Among with other changes (ptgen, image*.mk, base-files, ...) this
allows for a much more straight forward storage model on block based
devices.

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