update-rc.d: don't do anything if systemd.bbclass is inherited

We need the update-rc.d class to work when systemd is being used so that
packages that only have SysV init scripts still work.  However if a recipe
supports both we don't want to install SysV and systemd files under systemd.

To solve this, before doing real work in update-rc.d check if the systemd class
has been inherited and don't do anything if it has.

(From OE-Core rev: 0273a22fec3c9360df2510b759c5bf9af610551f)

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