In some cases it is practical to supervise as much as possible of
the barebox execution with a watchdog (or multiple watchdogs). This
patch provides an async poller for watchdog core framework which can
be enabled by the user and stores this configuration to nv.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>