Make menu more configurable: Ability to disable profile switch entry

New config.h entry (on by default):
    #define SUPPORT_PROFILE_SWITCH_MENU
Only show bluetooth menu item when bluetooth is needed at all
Show message to user if battery charge was detected
Make battery charge detection configurable

Battery charge detection behaves flaky if the battery
is quite empty: If you switch off the bike for a few hours
and come back, the voltage is quite often above the two volts threshold.

For LiFePo A123 cells the voltage drop occurs only
at the end when the battery is reeeaaly empty.
-> Manual reset is best here anyway.
Fix up logic for battery charge info message