systemd.bbclass: enhance checks for SYSTEMD_PACKAGES and SYSTEMD_SERVICE

after recipe parsing the following plausibilities are checked:
* ERROR if empty SYSTEMD_PACKAGES
* ERROR if SYSTEMD_PACKAGES entry does not contain '-systemd' AND
  package_name != ${PN}
* ERROR if SYSTEMD_PACKAGES entry contains '-systemd' but entry without
  '-systemd' (base pkg) does not exist
* WARNING if SYSTEMD_PACKAGES containing ${PN} to force package maintainers to
  split out systemd-specific files into *-systemd
* error if SYSTEMD_SERVICE(_<systemd-package>) is empty for one or more entries
  in SYSTEMD_PACKAGES

The code in populate_packages_prepend to set SYSTEMD_PACKAGES to ${PN} as
default was removed.

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