Add a script that installs a dmxknx installation with all dependencies on a Raspberry Pi with Archlinux ARM.

The script will automatically
* Create a "technik" user that runs FHEM and dmxknx
* Download & Install FHEM
* Download & Install HIDapi and usbdmx (FX5)
* Download & Install dmxknx
* Make service files for FHEM & dmxknx
 → Automatically restarts FHEM / dmxknx after 5 seconds if they crash
* Make the SD Card readonly so that power cuts won't break the filesystem

The script itself works and takes ~25 minutes to complete, FHEM is also working fine.
DMXKNX has not been tested yet.