technux-rpi: add basic image support

Add basic support for a Technux RPi image.
The image is heavily based upon the existing meta-raspberrypi layer.

To build the new image, meta-raspberrypi must be included in
bblayers.conf (see README.md for more info).

Change-Id: Idb098fe1f9d6e1dcd4c57d5bfb5473607b0a8591
Signed-off-by: Petter Mabäcker <petter@technux.se>