Use the WATCHDOG_USEC env variable to define the watchdog timeout

Use the WATCHDOG_USEC env variable to define the watchdog timeout

Also, set WatchdogSec to 5 seconds in boot-manager.service.in and
nsm-dummy.service.in.

The way this works is that systemd will take the WatchdogSec value,
convert it to micro-seconds and set the WATCHDOG_USEC environment
variable for the boot manager or nsm-dummy process. The boot manager
or NSM dummy then halves that value and creates the WatchdogClient
so that it calls sd_notify(0, WATCHDOG=1) twice in each
WATCHDOG_USEC interval. If WATCHDOG_USEC can't be parsed into a
number or if it is not set, the systemd watchdog timestamp is
never updated.

This is what systemd's requires services to do in order to support its
watchdog feature properly.