core: split out the core loop out of main()

core: split out the core loop out of main()

No real functional changes, just some rearranging to shorten the overly
long main() function a bit.

This gets rid of the arm_reboot_watchdog variable, as it can be directly
derived from shutdown_verb, and we need it only one time. By dropping it
we can reduce the number of arguments we need to pass around.