Arducopter: Changes to low voltage handling.  Move the Beeper control within CopterLEDS.  Create check to only turn on beeper if we are armed while low_voltage flag is set.  Stops endless beeping if using dual battery input, so that the APM is alive without the motor battery being plugged in.  Also, check the voltage state when arming, and if the voltage is good, then reset the low_voltage flag.  This will help with the same situation with a dual-power system which may have already set the low_voltage flag on booting without the motor battery attached.