wic: Fix usage of fstype=none in wic

wic: Fix usage of fstype=none in wic

This allows to specify partition with fstype=none in the wks file
to have partition created but without following mkfs. The none fstype
is in the list already but the usage is not documented.

Example;
part /data --ondisk mmcblk0 --fstype=none  --align 4096 --fixed-size 512

will create a partition, filesystem may be created manualy on the host
or target and data will be preserved if the device is reflashed using
same wks. Works with bmaptool and probably does not work with dd.
Use case is persistent filesystem/data between reflashing of the image.

Signed-off-by: Pavel Zhukov <pavel@zhukoff.net>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>