ar71xx has just one board name "wndr3700" for WNDR3700 V1/V2,
WNDR3800 and WNDR3800CH, whereas ath79 provides separate images for
the boards. So, update SUPPORTED_DEVICES to store the correct
ar71xx board names.
Fixes: FS#2510
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>