west/sign: Move from using partition label property

MCUmgr no longer uses DTS node property label to identify
slots for image upload and running application. It now uses DTS node
labels for that purpose.
This commit moves `west sign` from using label properties
to DTS node labels, same way the MCUmgr does.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>