geek/tos3-migration: update with changes introduced in TOS 5.2.1

We fixed postupdate hooks in TOS 5.2.0 and thus we no longer have to warn about Turris 1.x kernel not being updated as well as alternatives being ignored.

At the same time to fix issue with inaccessible router over the LAN Ethernet ports now reboot happens right at the end of migration. This is unconditional. This removes all notifications collected up to that time so we have to strongly suggest users that they should have notification sending configured to get all notifications generated during the update.