SW: Move diffutils + iw from GRMLBASE to GRML_* classes.

This allows using $EXIT_ON_MISSING_PACKAGES with GRMLBASE
but without have to use any of the GRML_* classes if building
a Debian/lenny based system. As iw/diffutils are missing on
lenny this would conflict $EXIT_ON_MISSING_PACKAGES as soon
as you've to use GRMLBASE.