This converts all fedora links in our samples to `mirrors.kernel.org`.
This mirror works best from around the world, so lets avoid the wild
mix of local mirrors and instead use kernel.org.
This mirror is also well-managed and properly funded, so we should not
run into too many problems with it.
That was mostly prepared already, the one last missing piece was
to use the default grub2 stage (previous commit) and now, with
this commit, to actually install the grub2 legacy bootloader via
the qemu assembler.
An example that takes an ostree commit and creates a qcow2 image
that is bootable. NB: the commit ids and the corresponding source
will need to be adjusted after having run the pipeline that will
create the ostree commit.
