Avoid using a single USART driver in more than one thread.

There are probably ways, but USART1 works fine if I do this.
Print debug over USART1.

The pin arrangement on the Discovery board is more convenient.
Set I2C lines as open drain.
Print time.
Check for errors.
Cast MSB to int16_t.
Increase TX buffer size.
Format print more nicely.
Fix (with a hack) problem with I2C not working.