Add Mynewt option to enable the watchdog

For Mynewt, if a watchdog driver is available, it is always used and feed
during a swap operation. Since the swap operation is able to resist resets,
the watchdog can stay disabled to preserve some flash space (watchdog
driver), with the side-effect that a swap might take longer because of
having to resume interrupted operations.

Signed-off-by: Fabio Utzig <utzig@apache.org>