fixed beeper -ON_USB for 1S setup, fixed beeper -SYSTEM_INIT

[RomanLut]

This submit fixes two issues:

- beeper -ON_USB silences beeper completely on 1S setup.
IS: getBatteryCount()<2 is used to check if battery is disconnected
SHOULD: getBatteryState() == BATTERY_NOT_PRESENT should be used instead.

- beeper -SYSTEM_INIT does not disable system init sound.
IS: SYSTEM_INIT flag is read from beeper PREFERENCES for unknown reason.
SHOULD: getBeeperOffMask() should be used

Note: ON_USB was described as silencing when USB is connected. Actually it silences when battery is disconnected (so FC is powered from USB).

Note: System init sound is not influenced by ON_USB because system init sound is played before battery sensor is configured - Ok.

Note: I guess that on some FCs battery sensor may (incorrectly) sense USB voltage, and thus battery sensor can detect 1S battery. Well, on these targets ON_USB will not work. From the beeper part, code is correct. If workaround is reqired, it should be in battery sensor code, not in beeper code: getBatteryState() == BATTERY_NOT_PRESENT should work correctly.

Sidenote: personally I would remove beeper PREFERENCES as redundand feature.