rilmodem: Add radio-settings driver

This driver adds support for setting the preferred radio technology.
It provides ofono support to set the preferred cellular radio (see
https://bugs.launchpad.net/ubuntu-system-settings/+bug/1211804).

Based on nemomobile implementation:

https://github.com/nemomobile-packages/ofono/tree/master/ofono

The original author of this code is Jussi Kangas
<jussi.kangas@tieto.com>.
build: Compile ril radio-settings module
ril: Create radio-settings interface
unit: Add tests for ril radio-settings parcels