pc-bios/s390-ccw: Link the netboot code into the main s390-ccw.img binary

pc-bios/s390-ccw: Link the netboot code into the main s390-ccw.img binary

We originally built a separate binary for the netboot code since it
was considered as experimental and we could not be sure that the
necessary SLOF module had been checked out. Time passed, the code
proved its usefulness, and the build system nowadays makes sure that
the SLOF module is checked out if you have a s390x compiler available
for building the s390-ccw bios. So there is no real compelling reason
anymore to keep the netboot code in a separate binary. Linking the
code together with the main s390-ccw.img will make future enhancements
much easier, like supporting more than one boot device.

Co-authored by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
Message-ID: <20241020012953.1380075-4-jrossi@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>