apm821xx: make it possible to update the dtb partition on the WNDR4700

Currently, the device-tree partition is marked as read-only.
Hence, userspace tools like mtd can't write into the partition.
This however will be necessary in case the DTB needs to be
updated.

This patch also adds the kernel.dtb image, so the compiled
DTB is exported as a file and available in the binary
directory along the firmware images.

Note: the u-boot does expects the dtb to be a uimage.

To update the dtb manually:
 1. copy the generated dtb to the router.
 2. mtd erase /dev/mtd2
 3. mtd write wndr4700.dtb /dev/mtd2

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/apm821xx/dts/wndr4700.dts

@@ -171,7 +171,6 @@
 			partition0,2@0x00180000 {
 				label = "device-tree";
 				reg = <0x00180000 0x00020000>;
-				read-only;
 			};
 
 			partition0,3@0x001a0000 {

target/linux/apm821xx/image/Makefile

@@ -22,6 +22,10 @@ define Build/dtb
 	$(call Image/BuildDTB,../dts/$(DEVICE_DTS).dts,$@.dtb,,--space $(DTB_SIZE))
 endef
 
+define Build/export-dtb
+	cp $(IMAGE_KERNEL).dtb $@
+endef
+
 ifeq ($(SUBTARGET),nand)
 
 define Image/cpiogz
@@ -186,13 +190,14 @@ define Device/WNDR4700
   BLOCKSIZE := 128k
   DTB_SIZE := 131008
   IMAGE_SIZE := 24960k
-  IMAGES := factory.img sysupgrade.tar
+  IMAGES := factory.img sysupgrade.tar kernel.dtb
   KERNEL_SIZE := 1920k
   KERNEL := dtb | kernel-bin | lzma | uImage lzma | pad-offset $$(BLOCKSIZE) 64 | append-fakerootfs
   KERNEL_INITRAMFS := kernel-bin | gzip | dtb | wndr4700-specialImage gzip
   IMAGE/factory.img := create-uImage-dtb | append-kernel | pad-to 2M | append-ubi | \
 		       netgear-dni | check-size $$$$(IMAGE_SIZE)
   IMAGE/sysupgrade.tar := sysupgrade-tar
+  IMAGE/kernel.dtb := export-dtb | uImage none
   NETGEAR_BOARD_ID := WNDR4700
   NETGEAR_HW_ID := 29763875+128+256
   UBINIZE_OPTS := -E 5
@@ -230,10 +235,6 @@ define Build/hdd-img
 	$(if $(CONFIG_TARGET_IMAGES_GZIP),gzip -9n -c $@ > $(BIN_DIR)/$(notdir $@).gz)
 endef
 
-define Build/export-dtb
-	cp $(IMAGE_KERNEL).dtb $@
-endef
-
 define Device/MyBookLiveDefault
   IMAGE_SIZE := 48m
   BLOCKSIZE := 1k