package/mender: update rootfs parts in mender.conf

The configuration options (RootfsPartA/RootfsPartB) must
provide a valid path to a block devices.

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>