ramips: remove factory image for TP-Link Archer C20 v1

Similarly to the Archer C2 v1, the Archer C20 v1 will brick when one
tries to flash an OpenWrt factory image through the TP-Link web UI.
The wiki page contains an explicit warning about this [1].

Disable the factory image altogether since it serves no purpose.

[1] https://openwrt.org/toh/tp-link/tp-link_archer_c20_v1#installation

Signed-off-by: Stijn Segers <[email protected]>

target/linux/ramips/image/mt7620.mk

@@ -981,6 +981,7 @@ define Device/tplink_archer-c20-v1
   TPLINK_HWID := 0xc2000001
   TPLINK_HWREV := 0x44
   TPLINK_HWREVADD := 0x1
+  IMAGES := sysupgrade.bin
   DEVICE_MODEL := Archer C20
   DEVICE_VARIANT := v1
   DEVICE_PACKAGES := kmod-mt76x0e kmod-usb2 kmod-usb-ohci \