This commit adds the necessary code in the dualboot stub (bootloader) to
let rockbox control the boot process. In particular, rockbox can now choose
if the next boot will be normal (boot rockbox or OF on magic key), to OF
or to updater.
The intents (to be added in follow-up commits) are:
1) Let the user more easily reboot to the OF. On some targets it is not trivial,
especially in USB mode.
2) Automatically reboot to updater when the user drop firmware.sb at the root
of the drive (currently, the user needs to do that in OF USB mode)
3) Document this OF magic