apm821xx: WNDR4700: fix broken sysupgrade, factory images

apm821xx: WNDR4700: fix broken sysupgrade, factory images

prepend-dtb got extended to handle the Meraki devices too,
the problem here was that the Netgear WNDR4700 expects an
u-boot header in front of the DTB, whereas Meraki devices
don't.

Since the header was dropped, the WNDR4700's uboot started
to complain:
  Bad Magic Number,it is forbidden to be written to flash!!

when flashing the factory.img since it expects an u-boot
header there.

Fixes: 5dece2d9355a ("apm821xx: switch over from DTB_SIZE to DEVICE_DTC_FLAGS")
Fixes: #13716
Reported-by: @kisgezenguz
Reported-by: Tamas Szabo
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(cherry picked from commit d6a11833ad67c33ad10dadf396f6c30bb44ef30f)