* [process-reboot-cause]Address the issue: Incorrect reboot cause returned when warm reboot follows a hardware caused reboot
1. check whether /proc/cmdline indicates warm/fast reboot.
if yes the software reboot cause file will be treated as the reboot cause.
finish
2. check whether platform api returns a reboot cause.
if yes it is treated as the reboot cause.
finish.
3. check whether /hosts/reboot-cause contains a cause.
if yes it is treated as the cause otherwise return unknown.
* [process-reboot-cause]Fix review comments
* [process-reboot-cause]address comments
1. use "with" statement
2. update fast/warm reboot BOOT_ARG
* [process-reboot-cause]address comments
* refactor the code flow
* Remove escape
* Remove extra ':'
In case of going from previous iteration of SONiC, and the last reboot
was hardware, REBOOT_CAUSE_FILE may not be present and the service may
throw an error.
