rework prebuilt download

Add addprebuilts command. This command looks for .mk files in
<root>/device/*/*/*.mk and searches these for a TARGET_ARCH = <some arch>.
Then all the needed host and target prebuilts are added to groups. In the
manifest the prebuilts need to get assigned to the correct groups of course.

Signed-off-by: Constantin Runge <uroc327@cssbook.de>
use my repo url

Signed-off-by: Constantin Runge <uroc327@cssbook.de>