wic: add 'none' fstype for custom image

wic: add 'none' fstype for custom image

It's not possible to set the label (of gpt entry) normally
when I want to use non-listed fstype as a rawcopy.

Example)
part ? --source rawcopy --ondisk mmcblk0 --label mypart --sourceparams file=mypart.raw

To resolve this problem, this patch addes a 'none' fstype
and ignore do_image_label on rawcopy (that actually set the partition label.)

Signed-off-by: JeongBong Seo <jb.seo@lge.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>