First attempt at usart communication

Setup USART3 for read and write
not tested
Fix USART communication

Basic USART communication via USART3 operational
TODO: connect BT module and work out communication protocol
Automatically connect USB in vagrant
Move USART init to bluetooth.c

All further code for bluetooth communication can be put in bluetooth.c
Set USART baudrate to 115200

Enables two-way communication between microcontroller and computer with
serial-bt- terminal