With the previous implementation, rebooting without registering a recognized
reboot mode would end up with U-Boot checking for a valid power-on reason, which
might result in the device turning off (e.g. with no USB cable attached and no
buttons pressed).

Since this approach is not viable (breaks reboot in most cases), the validity of
the reboot reason is checked (in turn, by checking that a warm reset happened,
as there is no magic) to detect a reboot and the 'o' char is recognized to
indicate that power-off is required. Still, that might be overridden by the
detection of usual power-on reasons, on purpose.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>