ath79: fix TL-MR3020 image metadata

Sysupgrading to ath79 from ar71xx currently fails because of mismatching
supported_devices. ar71xx is expecting "tl-mr3020" which is missing in
the ath79 image. Upgrading from ath79 is unaffected, as the image
contains the old string for ar71xx and the new one coming from the
device-tree.

Signed-off-by: David Bauer <mail@david-bauer.net>