calculate heading using gyro-only on boards without mag - idea by Cesco
added constants for gyro/acc/baro cal and fixed calibration to add /2
warning cleanup in drv_serial.c

git-svn-id: https://afrodevices.googlecode.com/svn/trunk/baseflight@426 7c89a4a9-59b9-e629-4cfe-3a2d53b20e61