ramips: Switch Teltonika RUT5xx to kernel GPIO-line watchdog driver

ramips: Switch Teltonika RUT5xx to kernel GPIO-line watchdog driver

The commit 04e91631e050 ("om-watchdog: add support for Teltonika RUT5xx
(ramips)") used the deprecated om-watchdog daemon to handle the GPIO-line
connected watchdog on the Teltonika RUT5xx.

But this daemon has massive problems since commit 30f61a34b4cf
("base-files: always use staged sysupgrade"). The process will always be
stopped on sysupgrades. If the sysupgrade takes slightly longer, the
watchdog is not triggered at the correct time and thus the sysupgrade will
interrupted hard by the watchdog sysupgrade. And this hard interrupt can
easily brick the device when there is no fallback (dual-boot, ...).

Signed-off-by: Sven Eckelmann <sven@narfation.org>