build: centralize fakeroot code

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>