sniper: Proper reboot mode and boot reason validation

sniper: Proper reboot mode and boot reason validation

With the previous implementation, rebooting without registering a recognized
reboot mode (despite registering the magic) 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).

This was designed to catch reboots that are actually intended to be power-off,
something that old Android kernels do, instead of properly turning the device
off using the TWL4030.

However, since this approach is not viable (breaks reboot in most cases), the
validity of the reboot mode magic is checked 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>