We already define it in Makefile.target.  But we need to avoid a
curious double negation in order to eliminate it.
Tested-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>