ramips: use nand_do_upgrade for netis WF-2881

WF-2881 sysupgrade image uses UBI rootfs, but still relies on
default_do_upgrade. Because of this, config backup is not restored after
sysupgrade. It can be fixed by switching to nand_do_upgrade and
sysupgrade-tar image. default_do_upgrade does not handle sysupgrade-tar
properly, so one should use factory image to upgrade from older version.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ramips/dts/mt7621_netis_wf-2881.dts

@@ -75,9 +75,22 @@
 		};
 
 		partition@140000 {
-			compatible = "denx,uimage";
 			label = "firmware";
 			reg = <0x140000 0x7e40000>;
+
+			compatible = "fixed-partitions";
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			partition@0 {
+				label = "kernel";
+				reg = <0x0 0x400000>;
+			};
+
+			partition@400000 {
+				label = "ubi";
+				reg = <0x400000 0x7a40000>;
+			};
 		};
 	};
 };

target/linux/ramips/image/mt7621.mk

@@ -628,10 +628,12 @@ define Device/netis_wf-2881
   BLOCKSIZE := 128k
   PAGESIZE := 2048
   FILESYSTEMS := squashfs
+  KERNEL_SIZE := 4096k
   IMAGE_SIZE := 129280k
-  KERNEL := $(KERNEL_DTB) | pad-offset $$(BLOCKSIZE) 64 | uImage lzma
   UBINIZE_OPTS := -E 5
-  IMAGE/sysupgrade.bin := append-kernel | append-ubi | append-metadata | \
+  IMAGES += factory.bin
+  IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
+  IMAGE/factory.bin := append-kernel | pad-to $$$$(KERNEL_SIZE) | append-ubi | \
 	check-size $$$$(IMAGE_SIZE)
   DEVICE_VENDOR := NETIS
   DEVICE_MODEL := WF-2881

target/linux/ramips/mt7621/base-files/lib/upgrade/platform.sh

@@ -49,6 +49,7 @@ platform_do_upgrade() {
 	netgear,r6260|\
 	netgear,r6350|\
 	netgear,r6850|\
+	netis,wf-2881|\
 	xiaomi,mir3g|\
 	xiaomi,mir3p)
 		nand_do_upgrade "$1"