systemd.bbclass: extract SYSTEMD_SERVICE properly

To set service files the following syntax is allowed:

SYSTEMD_SERVICE = <service_file>
or
SYSTEMD_SERVICE_{<systemd_package_name>} = <service_file>

A function get_package_var was introduced to handle all use-cases [1]
properly.

Build tested / buildhistory checks

[1] http://lists.linuxtogo.org/pipermail/openembedded-devel/2012-May/039870.html



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Acked-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>