core: add KExecWatchdogSec option

Rather than always enabling the shutdown WD on kexec, which might be
dangerous in case the kernel driver and/or the hardware implementation
does not reset the wd on kexec, add a new timer, disabled by default,
to let users optionally enable the shutdown WD on kexec separately
from the runtime and reboot ones. Advise in the documentation to
also use the runtime WD in conjunction with it.

Fixes: a637d0f9 ("core: set shutdown watchdog on kexec too")