Write new /boot/grub2/grub.cfg after activating the branding

/usr/share/grub2/themes/openSUSE/activate-theme only writes the info
about the theme to /etc/default/grub, but does NOT actually 'activate'
the new theme

An extra run to generate the grub config takes care of that
This discards the previous value of `res` and makes the log lie about what it logs in the next line.