olimex_a20_olinuxino_lime: rename board dir to remove _lime suffix

The files in board/olimex/a20_olinuxino_lime/ work with minimal or no
modifications for the Olimex A20-OLinuXino-Micro board, whose support is
going to be added in a later commit.

Rename the directory to clarify it's not restricted to the Lime anymore.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>