mvebu: fix broken console on WRT32X (venom)

mvebu: fix broken console on WRT32X (venom)

The console bootarg is being corrupted on boot, causing various issues
including broken sysupgrade.
Utilising the bootargs mangle patch from other targets, hardcode the console
arguments and fetch the rootfs from the bootloader.

Kernel command line: console=ttyS0,115200 root=/dev/mtdblock8

Bootloader command line (ignored): console= root=/dev/mtdblock8

Please cherry pick to 18.06 too

Signed-off-by: Michael Gray <michael.gray@lantisproject.com>
(cherry picked from commit 4fdc6ca31bda4aad2a65b8460b0caa5c60f87f41)