#13 Use bootclk

[Tobias Frost]

I'm using watchdog in an embedded target, where the target is in suspend most of the time, and as the monotonic clock is not advancing in suspend, the checks are not executed as often as configured.
The patch switches the waiting logic to BOOTCLK, which is ticking in suspend as well

Cheers,
tobi

[Paul Crawford]

Applied with added check that CLOCK_BOOTTIME is defined.