Old MikroTik devices have the RLE-encoded radio calibration data
directly stored in the art (hard_config) partition, without LZO
compression nor any preceding ERD magic bytes. This commit adds
a fallback for these devices.
Tested on the ath79 target with a MikroTik SXT 5nD r2 (SXT Lite5),
only locally --not yet merged upstream--.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>