u-boot fix to charge empty battery

[Philipp Hug]

I have 2 remaining issues:

* charging is only done when the LED is not active. Probably to make 
measurement of power more accurate.
-> Is this really necessary? It really slows down charging.

* revisions <6 don't blink the LED when charging, this isn't optimal, 
because can't see the difference between being switched off and charging 
silently.

Does anyone know how much current the LED use on GTA02v5?