bcm4908: enable Netgear R8000P bootloader image

bcm4908: enable Netgear R8000P bootloader image

This enables building BCM4908 "raw" image that can be flashed using
bootloader web UI. It requires serial console access & stopping booting
by the "Press any key to stop auto run".

It's easy to build vendor like CHK image but it can't be safely flashed
using vendor UI at this point. Netgear implements method called "NAND
incremental flashing" that doesn't seem to flash bootfs partition as
provided.

Above method seems to update vmlinux.lz without updating 94908.dtb. It
prevents OpenWrt kernel from booting due to incomplete DTB file. Full
Netgear R8000P support can be enabled after finding a way to make vendor
firmware flash OpenWrt firmware including the 94908.dtb update.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit d92a9c97bf3700e90af1d3c9157502af660365c0)