Rockbox

This fixes the low battery problem when booting. Up to
now you still could boot even if battery was definately
too empty to succeed this task.

Battery voltage is checked before disk spins up and if
voltage is below 3.05V the unit will be switched off
after showing a 'Battery empty!' message.
The warning threshold was rised to 3.5V, because
typically at this voltage you don't have much playing
time left.

And there is a voltage and a simple battery percent
level display in the 'Bootloader USB mode' so you can
see if you're save using the disk.

I didn't install it yet on my iHP140 because I'm a
littlebit afraid if I did anything wrong. But it
compiled fine and I could create a new firmware file by
using the scramble and mkboot tools. Theoretically this
should work but perhaps it would be better if somebody
with firmware flash access could test this (linus?).