watchdog: add kernel config parameter

Add DARMOK_WATCHDOG_TIMEOUT and DARMOK_WATCHDOG_KEEPALIVE as kernel config
parameter. DARMOK_WATCHDOG_TIMEOUT is reboot timeout and each
DARMOK_WATCHDOG_KEEPALIVE time a pong message is sent. If
DARMOK_WATCHDOG_KEEPALIVE is 0, then automatic watchdog is disabled and
the user space must handle watchdog.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>