wic: include partition label in fs image file name

Not including the label image but having multiple partitions of the same
type, the image file of one partition would overwrite the other.

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>