Buzzer redesign
Made new buzzer implementation. It is now easier to add new buzzer alerts by just adding buzzer("alert sequence") function call. Also adding new "tones" is made easy.  Implementation is light doesn't use much CPU load. Downside of this is that it doesn't support real music because it just outputs square wave to piezo (Morse code style. Tone doesn't change.). Sin wave creation would take much more CPU load.

New additions:
- Beeps when arming or disarming the board. (different arming beep when GPS is fixed)
- Continuous warning beeps when board is armed and throttle is low (motors not spinning option is enabled)
- Board beeps when GPS has found satellites and is in fixed state (ready to flight)
- TX AUX channel beep can be used for counting how many satellites has been found (up to ten beeps)
- Now it's beeping SOS Morse code when armed and TX is lost -> autolanding
- New battery alert level (default 3,5V per cell). When reached buzzed start beeping in slow cycles. (There are still that old battery min level 3.3V when buzzer beeps in fast cycles and lipo is almost empty and dead. Used more than recommended 80% of capacity)
New stable version release; Addition of atail4 model; Fix for SBUS receiver scaling; Spektrum satellite bind support; Fix in softserial code
Inspection fixes
Merge pull request #220 from jaahaavi/Buzzer

Buzzer redesign