wic: mountpoint is an optional argument

wic: mountpoint is an optional argument

According to wic documentation partition mount point is an optional
argument. Skipping mount point also makes sense in certain
configurations when one needs to specify a partition that is not mounted
by the running system, such as a recovery or a mirror partition (in dual
rootfs setups).

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>