Take it from me, you DON’T want to power-cycle watchdog the VUSB input pin. That will actually have the opposite effect, guaranteeing your Boron will be dead-forever-until-human on the first reset. They say use the EN pin but I think the only real way to do a watchdog for Boron is to make a robot that physically pulls it out of your pin sockets and pushes it back in, or else the leakage current from even one externally powered UART RX pin will prevent boot. See my thread here: (Video) New Boron FAILS to reboot on VUSB power cycle - external watchdog incapable of resetting device
Particle said over a month ago that there was going to be a technical memorandum/application note on Boron watchdog implementation, which to my knowledge does not yet exist: (Video) New Boron FAILS to reboot on VUSB power cycle - external watchdog incapable of resetting device
By far the best results (that is, the only acceptable results) I have been able to have with Boron are 1) MANDATORY usage of 1.3.1-rc1 and NOT newer, and 2) no watchdog.