ath79: fix sysupgrade from ar71xx for WNDR3700 V2 and WNDR3800(CH)

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>
(cherry picked from commit fc44a8481cbf317febaf27a550e0c9fa49be68d5)

target/linux/ath79/image/generic.mk

@@ -632,7 +632,7 @@ define Device/netgear_wndr3700v2
   NETGEAR_BOARD_ID := WNDR3700v2
   NETGEAR_HW_ID := 29763654+16+64
   IMAGE_SIZE := 15872k
-  SUPPORTED_DEVICES += wndr3700v2
+  SUPPORTED_DEVICES += wndr3700
 endef
 TARGET_DEVICES += netgear_wndr3700v2
 
@@ -652,7 +652,7 @@ define Device/netgear_wndr3800
   NETGEAR_BOARD_ID := WNDR3800
   NETGEAR_HW_ID := 29763654+16+128
   IMAGE_SIZE := 15872k
-  SUPPORTED_DEVICES += wndr3800
+  SUPPORTED_DEVICES += wndr3700
 endef
 TARGET_DEVICES += netgear_wndr3800