Changed the IMAGEBUILDER_URL and added the backported devices

The IMAGEBUILDER_URL now points to my OpenWrt AA repo with some newly backported devices and some additional bugfixes. Currently I only have the ar71xx imagebuilder available.

Additionally there are some extra targets added to the Makefile for the new devices.

updated dev.openwrt.piratebox.de to beta.openwrt.piratebox.de