De-couple battery code.

This fixes a bug where buzzerFreq could be uninitialised before it's
use.
This improves performance by only checking whether to sound the battery
alarm after the battery voltage has been recalculated.
There were unused battery beep codes which have been deleted to save
code size.
Configuration structure extracted from mw.h into config.h.
Moved mixer configuration structures into flight_mixer.h.
Added a build_config.c in order to decouple pwm_common.h from config.h.
Finally, battery configuration values now live in a batteryConfig
structure which means that battery.c does not depend on config.h and all
of it's dependencies.

Fixed the use of plurals on defines that were not collection objects.
PIDITEMS and CHECKBOXITEMS are now PID_ITEM_COUNT and
CHECKBOX_ITEM_COUNT.