On the Taijiuino, need to set 13 as output on startup.

Arduino does it on boot because it toggles the LED. Taijiuino
doesn't have a LED.
Add configurable offset to output PPS

TICs can usually only measure in one direction, so an offset greater than
the maximum range where we qualify the PPS will keep the sign of the
time difference always positive
Commented code for dealing with Resolution-T sawtooth
"make reset" rule
``` command to soft-reset from the serial console
Wait for the serial to come up in console_init()
half-tick offset boosts the PLL gain close in to the origin

without this, there's a tendency to "wander" within a 100ns corridor,
bouncing off of both sides. With it, dithering tends to keep things
within 3-4 84MHz ticks of correct, with any noise at a much higher
frequency.