Enhance switch agent to reboot into ONIE install mode

[hardikdr]

Summary

Enhance the switch agent so it can trigger a switch reboot into ONIE install mode, allowing the device to re-enter the ONIE installer discovery flow and reinstall SONiC.

Basic example

Proposed high-level flow:

1. Agent configures the grub-entry to enter into the install mode.
2. Agent reboots the switch.
3. After reboot, ONIE starts in install mode, enters installer discovery, and attempts to locate and run a SONiC installer via DHCPv6.

Motivation

Today the agent can manage switch state while SONiC is running, but it cannot trigger a full reprovisioning workflow through ONIE.
We need this capability to support cases where a switch must be reinstalled remotely without manual console access.

[hardikdr]

From @byteocean, on setting the grub entry.

sudo grub-editenv /host/grub/grubenv set next_entry=ONIE
sudo grub-editenv /mnt/onie-boot/grub/grubenv set onie_mode=install

sudo reboot

[Giluerre]

I strongly believe that part of the issue is that we need to create a mount point for the ONIE partition and mount it, right?