wic: fix wrong attempt to create file system in upartitioned regions

wic: fix wrong attempt to create file system in upartitioned regions

The kickstart parser defaults fstype to "vfat". This leads to an attempt
to create an empty file system even for regions configured with "--no-table"
if used without fstype when no --sourceparams given.

The fix tests for fstype "none" or no_table in Partition prepare method.
This will omit the file system creation an the potential error for small
region with --no-table option.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>