Add support for presence detection, actual and remaining battery capacity

The battery monitor firmware now has support for detecting battery presence and reporting actual and remaining battery capacity.

This patch removes the current support for battery presence detection, relying in the battery monitor to provide this information instead. In adding reporting for full capacity and current capacity, the variable used for current state of charge is renamed to avoid confusion.

Signed-off-by: Martyn Welch martyn@welchs.me.uk