When the board is on console, a way is added to manually power off the
board, on a long power key press (4s).

This enables to be able to poweroff the board whatever the state, and is
the only manual way (no mechanical possibility).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>