Remove inactive devices

Remove device which are EOL, unused or do not work.

As discussed during the SAT call:

- rpi2 :
 -- will not work for IVI due to the CPU
 -- use the rpi3 instead

- joule :
 -- EOL for ~1y, no longer available for purchase, no way to test.

i.MX6:
Remove currently unmaintaned boards. Can re-add on request.
- imx6qsabrelite:
 -- replaced with uprev'd imx6qsabreauto -> imx6qdlsabreauto
- nitrogen6x:
 -- no thud branch upstream.

Bug-AGL: SPEC-2323

Change-Id: I28a70dcb7b48b6a9396d3003cc34d4c1f6570eca
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>