configure: Let SLOF be initialized by ./scripts/git-submodule.sh

The git-submodule.sh script is called by make and initialize the
submodules listed in the GIT_SUBMODULES variable generated by
./configure.

SLOF is required for building the s390-ccw firmware on s390x, since
it is using the libnet code from SLOF for network booting.

Add it to the GIT_SUBMODULES when building the s390-ccw firmware.
Reported-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Suggested-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200615074919.12552-1-f4bug@amsat.org>
[thuth: Tweaked the commit message a little bit]
Signed-off-by: NThomas Huth <thuth@redhat.com>

.travis.yml

@@ -496,7 +496,6 @@ jobs:
         - CONFIG="--disable-containers --target-list=${MAIN_SOFTMMU_TARGETS},s390x-linux-user"
         - UNRELIABLE=true
       script:
-        - ( cd ${SRC_DIR} ; git submodule update --init roms/SLOF )
         - BUILD_RC=0 && make -j${JOBS} || BUILD_RC=$?
         - |
           if [ "$BUILD_RC" -eq 0 ] ; then

configure

@@ -6533,6 +6533,11 @@ if test "$cpu" = "s390x" ; then
   write_c_skeleton
   if compile_prog "-march=z900" ""; then
     roms="$roms s390-ccw"
+    # SLOF is required for building the s390-ccw firmware on s390x,
+    # since it is using the libnet code from SLOF for network booting.
+    if test -e "${source_path}/.git" ; then
+      git_submodules="${git_submodules} roms/SLOF"
+    fi
   fi
 fi