[aprssig] OpenTracker USB and Tracker2 updates

[Scott Miller]

I know some folks don't follow the groups for these devices, so I 
thought I'd mention here that there's a significant update available. 
The T2 updates are mostly maintenance issues - lots of restructuring and 
bug fixes - but the OTUSB update fixes a problem with the USB port being 
rendered unusable in certain configurations.

Just run otwincfg and use the 'web' button to update.

Another tweak to the OTUSB is improved temperature sensor accuracy.  The 
sensor is built in to the MCU and takes a lot more math to get good data 
from than the older external sensors, but it's working great now.  After 
a single-point calibration (same as all the other OpenTrackers) I didn't 
get more than a 1-degree C error from -40C to +50C in a calibrated 
temperature chamber, and not more than a 3-degree error from -195C to 
+80C, though it should be noted that it didn't stay running long at 
-195C (that's -319 Fahrenheit) and the insulation started cracking off 
the cables.  Recovered just fine when it warmed up a bit, though.

All ADC readings are now corrected against an on-chip bandgap reference, 
which helps compensate for regulation errors and ripple on the +5v 
supply, so the voltage reading should be more accurate, too.

Scott
N1VG