Add support to autorun rpmsg-demo when system boot.

The main changes are as follows:
* Introduce a SysV-style initscript to control the starting, stopping
  and restarting of rpmsg-remote daemon.
* Install the initscript with the help of update-rc.d class, thus
  rpmsg-remote can start automatically after system boot.
* Introduce a configuration XEN_DOM0LESS_DOM_LINUX_DEMO_AUTORUN to
  set if autorun rpmsg-demo when building the target image.
  And config it to "1" as default for virtualization stack.
  To disable it, build the stack using the following command:

      XEN_DOM0LESS_DOM_LINUX_DEMO_AUTORUN=0 \
          kas build v8r64/meta-armv8r64-extras/kas/virtualization.yml

* The autorun works only if XEN_DOM0LESS_ZEPHYR_APPLICATION is set as
  "zephyr-rpmsg-demo".

Issue-Id: SCM-4876
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Jiacheng Tang <jiacheng.tang@arm.com>
Change-Id: I49b430e86828910f8a73f2be4789493692bf11a9