USB suspend and resume to micropower mode actually works! Current
battery voltage is reported as ASCII value, in mV, at 1 Hz.

There are still small issues with occasional data coming through 2 at a
time at 0.5 Hz, along with a fluctuating input current drop for 1 s
suggesting something up with the USB spuriously suspending, instantly
waking up + resetting, then returning 2 lines when the next printf()
reports data.