modifications to lvc rampdown computations

[EigenMania]

I'm proposing a very slight modification for how the LVC rampdown computations are performed. I changed LVC_RAMP_DOWN_OFFSET_PERCENT to reflect the battery percentage when LVC rampdown starts, and the LVC rampdown ends when 0% battery is reached (the padded value), thus protecting the battery a bit more from discharge at configured LVC (and lining up with the displayed battery percentage).

Also moved the defines from battery.c into fwconfig.h and added some more detailed comments / examples for the battery math.

[timothymiller]

Looks good to me. Ship it!

[danielnilsson9]

Seems like a reasonable change, thanks!