rhel: allow passing more flags to configure, fedora

rhel: allow passing more flags to configure, fedora

Define a variable _ovs_config_extra_flags to allow passing more flags to
configure when building OVS kmod RPM. For example, to build with a
non-standard openssl and add an RPATH, use the following command

make rpm-fedora-kmod RPMBUILD_OPT='-D "_ovs_config_extra_flags
--with-openssl=<path to your openssl header> LDFLAGS=\"\${LDFLAGS} -Xlinker
-rpath=<path to your openssl lib>\""'

Signed-off-by: Martin Xu <martinxu9.ovs@gmail.com>
CC: Greg Rose <gvrose8192@gmail.com>
CC: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Reviewed-by: Markos Chandras <mchandras@suse.de>