ramips: mt7621: ubnt-erx: allow sysupgrade from master

Currently it's not possible to downgrade from master:

 Device ubiquiti,edgerouterx not supported by this image
 Supported devices: ubnt-erx

So fix it by adding a DTS based device name from master into
SUPPORTED_DEVICES list.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ramips/image/mt7621.mk

@@ -488,6 +488,7 @@ define Device/ubnt-erx
   KERNEL_INITRAMFS := $$(KERNEL) | ubnt-erx-factory-image $(KDIR)/tmp/$$(KERNEL_INITRAMFS_PREFIX)-factory.tar
   IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
   DEVICE_TITLE := Ubiquiti EdgeRouter X
+  SUPPORTED_DEVICES += ubiquiti,edgerouterx
 endef
 TARGET_DEVICES += ubnt-erx
 
@@ -496,6 +497,7 @@ define Device/ubnt-erx-sfp
   DTS := UBNT-ERX-SFP
   DEVICE_TITLE := Ubiquiti EdgeRouter X-SFP
   DEVICE_PACKAGES += kmod-i2c-algo-pca kmod-gpio-pca953x kmod-i2c-gpio-custom
+  SUPPORTED_DEVICES += ubiquiti,edgerouterx-sfp
 endef
 TARGET_DEVICES += ubnt-erx-sfp