Boot into ONIE and reinstall SONIC using Controller

[hardikdr]

Summary

Implement a bootstrap flow that configures a switch from inside SONiC (via the Agent) so the next reboot enters the ONIE shell. Once in ONIE, the controller detects ONIE mode and executes onie-nos-install to reinstall using the desired installer/image URL.

Motivation

Eliminates manual console/boot-menu steps to reach ONIE for reimaging.

[maxmoehl]

Rebooting into ONIE can be achieved via grub-reboot:

grub-reboot --boot-directory=/host ONIE

It will then automatically fall into the installer mode in ONIE and try to install an image using the usual discovery methods.

[maxmoehl]

When a switch boots into the ONIE environment it tries to discover the onie-installer URL from which to fetch the OS image which then gets installed onto the switch. This uses the DHCP option boot file URL (code 59) which FeDHCP already serves as part of its httpboot plugin. It also offers a static URL variant which should be enough to get us started. We should spend some time investigating if that plugin is already sufficient or if we need an additional one for this use-case.

[hardikdr]

Closing this one, in favor of a new approach of using DHCPv6 instead.