WIP: turris1x-btrfs: refactor package and migration script

Please @jschlehofer review package changes. @vmyslivec please review script changes.

hardware/turris1x/turris-btrfs/files/kernel-install

-#!/bin/sh
-if [ "`cat /proc/mounts | grep '^/dev/mmcblk0p2 / btrfs'`" ]; then
-	mkdir -p /boot/tefi
-	mount -t vfat /dev/mmcblk0p1 /boot/tefi
-	cmp /boot/zImage /boot/tefi/zImage || cp /boot/zImage /boot/tefi/zImage
-	cmp /boot/fdt /boot/tefi/fdt || cp /boot/fdt /boot/tefi/fdt
-	umount /boot/tefi
-fi