configs/olimex_a20_olinuxino: switch to extlinux.conf

a10_olinuxino is not switched to extlinux.conf, so the broken symlinks
are replaced by the previous content from a20_olinuxino.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>