build: centralize fakeroot code

This patch moves the fakeroot code required by some devices to
`image-commands.mk`.

Create the fakeroot on the fly by using the undocumented -s (skip copy)
parameter of mkimage.

Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>
[remove unused NETGEAR_KERNEL_MAGIC, remove workarounds to have a dummy
rootfs for mkimage]
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/image/mt7620.mk

@@ -58,7 +58,7 @@ define Device/ex2700
   BLOCKSIZE := 4k
   IMAGE_SIZE := $(ralink_default_fw_size_4M)
   IMAGES += factory.bin
-  KERNEL := $(KERNEL_DTB) | uImage lzma | pad-offset 64k 64 | append-file netgear-fake-uImage-hdr.bin
+  KERNEL := $(KERNEL_DTB) | uImage lzma | pad-offset 64k 64 | append-uImage-fakeroot-hdr
   IMAGE/factory.bin := $$(sysupgrade_bin) | check-size $$$$(IMAGE_SIZE) | \
 	netgear-dni
   DEVICE_TITLE := Netgear EX2700
@@ -71,7 +71,7 @@ define Device/wn3000rpv3
   DTS := WN3000RPV3
   BLOCKSIZE := 4k
   IMAGES += factory.bin
-  KERNEL := $(KERNEL_DTB) | uImage lzma | pad-offset 64k 64 | append-file netgear-fake-uImage-hdr.bin
+  KERNEL := $(KERNEL_DTB) | uImage lzma | pad-offset 64k 64 | append-uImage-fakeroot-hdr
   IMAGE/factory.bin := $$(sysupgrade_bin) | check-size $$$$(IMAGE_SIZE) | \
 	netgear-dni
   DEVICE_TITLE := Netgear WN3000RPv3