ubiupdatevol: add a --skip option

ubiupdatevol: add a --skip option

This already has a --size option for controlling how many bytes to read
from the input.  Add a --skip option to control the offset into the input
too.  This way people don't have to do `dd | ubiupdatevol`.

While we're here, I've fixed the types used with args.size and the read
loop so that they can hold the right sizes (like setting a 32bit+ size).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>