ipq40xx: fix sysupgrade image for Netgear LBR20

ipq40xx: fix sysupgrade image for Netgear LBR20

This fixes the sysupgrade image generation for the LBR20 as before
updating resulted in a brick which needed to be recovered using
nmrpflash.

UART log of a bricked unit:
Loading DNI firmware for checking...
Loading firmware 1 ...

NAND read: device 0 offset 0xa600000, size 0x20000
 131072 bytes read: OK

NAND read: device 0 offset 0xa600000, size 0x380000
 3670016 bytes read: OK
rootfs imge header corrupted !
Loading firmware 2 ...

NAND read: device 0 offset 0xa600000, size 0x20000
 131072 bytes read: OK

NAND read: device 0 offset 0xa600000, size 0x380000
 3670016 bytes read: OK
rootfs imge header corrupted !

Comparing the partition contents of a bricked and a working unit showed
that on the bricked one the fake uImage header was missing. The UBI
partition also showed significant changes. Both are fixed when the
base DniImage receipt is used.

Signed-off-by: Christoph Krapp <achterin@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20558
Signed-off-by: Robert Marko <robimarko@gmail.com>