u-boot fix to charge empty battery

[Werner Almesberger]

Philipp Hug wrote:
> the problem here is, that even 500mA is not enough to boot. at least  
> that's what I experienced.

When I did the measurements for the wait_for_power design in u-boot,
I never came even close to needing 500mA to load and start the kernel.
So I would suspect that, if you're indeed drawing more than 500mA and
this is not just the shutdown timer, the kernel turns on something it
should have left off in that case.

> but maybe this was also only related to vsys going below it's threshold  
> and initiating shutdown timer.

I think this one can explain quite a lot of power-on problems that
have been misinterpreted before. I also saw it a number of times,
but simply attributed it to batteries being too weak and didn't
even probe any further.

> ( we can't start GSM early in u-boot anymore)

It's just a few seconds anyway :)

> Better idea would probably be to charge to some level where we'll  
> survive the boot process.

But that's basically what wait_for_power does. What I missed is
that timer. I'm not sure how. Maybe it's related to what happens
when plugging in USB. I normally "plugged" USB by turning USB
power on and off, without a mechanical insertion.

- Werner