systemd.bbclass: helper class for recipes with systemd units

systemd.bbclass: helper class for recipes with systemd units

This class adds postinst/prerm scripts to start/stop/enable/disable the services
as relevant, and some magic to ensure the service files are installed.

Based on (but not the same as) the systemd.bbclass in meta-systemd, so thanks to
the following for their work there:

Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Khem Raj <raj.khem@gmail.com>
Martin Jansa <Martin.Jansa@gmail.com>
Andreas Müller <schnitzeltony@googlemail.com>
Koen Kooi <koen@dominion.thruhere.net>

(From OE-Core rev: f4bf51612f8be1d3dd340fc456f3fa08fcfa34ef)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>