Closes#1882.
Whenever power button is held long enough ("force shutdown"), mcu sysmodule
stores a flag in free reg 0. It will clear it next boot.
During that next boot, if that flag was set and if CFG_BOOTENV.bit0 is set
(warmboot/firm chainload, i.e. not coldbooting), then main() will simulate
a "power button held" interrupt (after upgrading mcu fw if necessary -- it
will reboot console after if it has upgraded mcu fw, I guess that's one of
the reasons the flag is there). This obviously cause other processes to initiate
a shutdown.
In the case of autoboot, ns will panic when this happens.
cc46e16d66