Move the handling of the roms/SLOF submodule out of the main Makefile,
since we are going to remove submodules from the build process of QEMU.

Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>