[coreboot] buildrom: Fix ": Unsupported scheme." error message and a typo

[Peter Stuge]

Cristi, note that this was the cause of the error message when I
tested your code too. The executed command is:

wget "" -P /home/stuge/co/kvm/sources http://kernel.org/pub/linux/kernel/v2.6//linux-2.6.22.2.tar.bz2

And of course wget can't download an empty string.

Makefile variables shouldn't have quotes. I can't commit to the
buildrom repo so the commiter could change this to := at will, and
I'd be fine with that too. :)

//Peter