Platform plugin for Fast-reboot/warm-reboot

[santhosh-kt]

- What I did
Added support for calling a platform specific plugin in fastboot, warmboot
- How I did it
Changes applied to 'scripts/fast-reboot'
- How to verify it
Write a platform plugin script and execute fast-reboot command
Plugin script will be executed and then reboot will be invoked
- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

[yxieca]

Can you give an example of how this platform plugin will be used, and why?

[Arul-Jeniston-Mc]

• Dell hardware stores all reset/reboot reasons (including cold and warm cpu resets) in a register. This hardware register value persists till next reset/reboot occurs.
• As the hardware doesn't have a way to detect fast-boot, following the fast-boot, hardware reset-cause register would end up having the previous reset reason value which is a stale value.
• This plugin is added to find this situation. Otherwise, following fast-boot, the reset reason API (that reads the data from hardware) returns incorrect reset reason.

For example:-

1. Run 1: User power-cycled the switch. .Hardware reset-cause register has value 0x11 which means power-cycle.
2. Run 2: reboot command executed from SONiC. Hardware reset-cause register value changed to 0x55 which means warm-cpu reboot.
3. Run 3: hardware watchdog resets the switch. Hardware reset-cause register value changed to 0x33 which means watchdog reset.
4. Run 4: fast-boot command executed from SONiC. Hardware reset-cause register value remains un-changed as 0x33 which means watchdog reset.
5. Run 5: show reboot reason call reset reason API which refers the hardware and returns watchdog reset. This is incorrect. The above change addresses this problem.

[yxieca]

retest this please

[santhosh-kt]

Please include this change for 201811 branch also.