[rilmodem] Configure the tech preference only once

Information if the configuration has been done or not cannot be
stored to same file where configuration is read from due the lack
of priviledges. This change makes ofono to store the information
to same place as any other setting stored by ofono.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #130 from jkangas/next

Request for pulling storing the configuration info to ofono storage directory to next branch