imx6: remove not longer supported aristainetos boards

Removed aristainetos2, 2b, 2b-csl. This boards have been
recalled and destroyed.

Adapt board code to remove stuff not needed anymore.

Fix checkpatch warning, remove fdt_high and initrd_high
from default environment.

Signed-off-by: Heiko Schocher <hs@denx.de>
zu remove