wic: check that filesystem is specified for a rootfs partition

wic: check that filesystem is specified for a rootfs partition

We explicitly check for --fstype if no source was provided for a
partition. However, this was not the case for rootfs partitions. Make
sure to raise an error if filesystem was left unspecified when preparing
a rootfs partition image.

(From OE-Core rev: b8c35fcad57810a87aa25ebeb533adf286eed565)

(From OE-Core rev: dc4d1cfc4d6bd01432857448f66af5492cad7574)

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>