ath79: image: add supported string for routerstations and ja76pf2

Now that the md5 check is fixed and metadata present, sysupgrade on
ar71xx will complain about device not being supported by the image.
Since the cause is not matching strings for supported devices add them
accordingly.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
(cherry picked from commit a45cf75eca)

target/linux/ath79/image/generic-ubnt.mk

@@ -218,6 +218,7 @@ define Device/ubnt_routerstation
   UBNT_TYPE := RSx
   UBNT_CHIP := ar7100
   DEVICE_PACKAGES += -swconfig
+  SUPPORTED_DEVICES += routerstation
 endef
 TARGET_DEVICES += ubnt_routerstation
 
@@ -227,6 +228,7 @@ define Device/ubnt_routerstation-pro
   UBNT_BOARD := RSPRO
   UBNT_TYPE := RSPRO
   UBNT_CHIP := ar7100pro
+  SUPPORTED_DEVICES += routerstation-pro
 endef
 TARGET_DEVICES += ubnt_routerstation-pro
 

target/linux/ath79/image/generic.mk

@@ -479,6 +479,7 @@ define Device/jjplus_ja76pf2
   KERNEL := kernel-bin | append-dtb | lzma | pad-to $$(BLOCKSIZE)
   KERNEL_INITRAMFS := kernel-bin | append-dtb
   IMAGE_SIZE := 16000k
+  SUPPORTED_DEVICES += ja76pf2
 endef
 TARGET_DEVICES += jjplus_ja76pf2