base-files: add reboot only button handler

base-files: add reboot only button handler

For devices such as BTHOMEHUBV5A with both reset and restart buttons,
its easily accessible restart button has been assigned to KEY_POWER
power script to poweroff preventing accidental (or malicious) factory
resets by KEY_RESTART reset script. However an easily accessible button
immediately powering off the device is also undesirable.

Fixes: FS#1965
Signed-off-by: Alan Swanson <reiver@improbability.net>
Signed-off-by: Petr Štetiar <ynezz@true.cz> [long line wrap]