when battery_bench is run
exports a file in the rockbox directory called 'battery_levels.default'
if the user wants their own levels they can rename the file battery_levels.cfg
and it will be loaded at boot
some minimal error checking is performed prior to using the values
added manual entry
Change-Id: Ia0126faced0c7229fcf8385a1bcb584b5a9dc378
affects all hiby targets, fiiom3k, and ibasso dx50/dx90
As well as deduplicating a small pile of code, this also implements
hysteresis so we're not doing a sysfs read/lookup multiple times
back-to-back every time the power management tick fires.
Change-Id: I2f7672acbb36341becf67e07960c24c681270d09
