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

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: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200615074919.12552-1-f4bug@amsat.org>
[thuth: Tweaked the commit message a little bit]
Signed-off-by: Thomas Huth <thuth@redhat.com>