configs/olimex_a20_olinuxino_lime: new board

This commit adds a new defconfig for the Olimex A20 OLinuxino Lime
board, based on the 4.x mainline kernel.

Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>