Partial revert "configs/olimex_a20_olinuxino: switch to extlinux.conf"

Old scripts are still used by a10_olinuxino & a20_olinuxino_micro

Fixes olimex_a20_olinuxino_micro
see https://gitlab.com/buildroot.org/buildroot/-/jobs/1478738568



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