twi: Fix some little flaws.

This makes the twi_add_frame function more intuitive, sets more flags
properly, and stops entering master receive mode for writes.
usart: Fix annoying comments.
gyro: Make the averaging function actually work.
Makefile: Add gyro, baro, and mag.
baro: Add some read and calibration code.
Add some control code.

control_handler is getting merged into control_timer in a bit.
esc: Fix comments.
baro: Fix syntax errors.
twi: Handle null flag cases.