libgudev: remove systemd dependency

Since version 3.1.3 eudev doesn't provide internal libgudev,
so this package can now be used as an ordinary stand-alone
package.

[Peter: add BR2_PACKAGE_HAS_UDEV dependency]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>