Pico power consumption down to 0.02mA

Firmwares from this one on (so 1v81 and later) are now able to properly power down USB when in Deep Sleep.

This drops power consumption from 0.25mA down to 0.02mA - so a pretty massive change that should really help with battery life. For instance a single CR2032 watch battery should now power the Pico for around a year.

To enable it, all you need is the usual setDeepSleep:

setInterval('digitalPulse(LED1,1,10)',5000);
setDeepSleep(1);

And unlike the Original board, the Pico's wake up timer is accurate to 1/32768 of a second, so it can re-enter deep sleep even when there are relatively short intervals (the original board could only handle this if the interval was greater than around 2 sec).

Let me know if you hit any problems - tests so far have been good though.

The board itself is capable of getting even lower power consumption, so hopefully a few more firmware tweaks and we can get under 0.01mA.