[U-Boot] Force setting of ethact

[DUNDA Matthias]

> Why don't you use "ethprime" ?
>
> > What's the intended/desired function of ethact, and how should it be
> initialized? Even using CONFIG_ETHPRIME has no effect.
>
> These variabes are documented in the README.

Yes, I read that! That's why I used CONFIG_ETHPRIME in my board header file to do the trick - maybe my env was corrupted, or something - I don't know, why it didn't show up by default... I had to setenv it by hand.

Thanks.
Matthias