systemd.bbclass: make easier handle service enabling

This generates the postinst/prerm/postrm routines for the package
specified in SYSTEMD_PACKAGES avoding code duplication and
centralizing the handling of it for easy maintainence work.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>