rampis: mt7621: Edgerouter-X disable toggling of kernel slots

rampis: mt7621: Edgerouter-X disable toggling of kernel slots

Uboot selects which kernel slot to boot based on a flag in the factory
mtd partition. Patch ubnt.sh to ensure always flash to kernel1 slot and
update flag if required.

Signed-off-by: Tim Lunn <tim@feathertop.org>
Tested-by: Mauri Sandberg <maukka@ext.kapsi.fi>
Link: https://github.com/openwrt/openwrt/pull/15194
(cherry picked from commit 5e355f1f9084c1b6de3151fbcde948d736292d6c)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>