Make it possible for target to provide voltage, percentage and time to
empty values. The voltage measurement is nice to have in debug menu even
if the actions are taken only based on percentage. Perform battery level
estimation based on voltage only if percentage is not available.
Use time to empty based on actual power consumption. This makes the
estimated runtime displayed in Rockbox Info to react to backlight
setting. The bq27000 updates time to empty estimate every 5.12 seconds
so it is possible to see the estimate with backlight off on the screen
if user enters Rockbox Info, activates hold switch, waits 11 seconds and
releases the hold switch.
Change-Id: Iafe3fa0fb334e3428e3a0ad05b2c020d208dc934
841e704fc3