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.
Add config option to autoboot into 3DS and DSi homebrew menu, without
going through Home Menu (nor launching it).
For 3DS homebrew, this requires homebrew built with libctru v2.0.0 or
later (v2.0.0 was released 2.5y ago).
We simulate a "reboot into title" to achieve this. This being said, when
launching stuff like Pokemon US/UM on O3DS, Home Menu reboots into
itself and not the game directly. This will cause Home Menu to crash if
you use this feature and configure it to use a non-default memory layout
(but if you don't, Home Menu will work just fine).
