armv8r64-extras/demo: Add support nginx-docker-demo autorun

Add support to autorun nginx-docker-demo when system boot.

The main changes are as follows:
* Introduce a SysV-style initscript to control the starting, stopping
  and restarting of nginx docker container.
* Install the initscript with the help of update-rc.d class, thus
  nginx docker container can start automatically after system boot.
* The autorun works only if XEN_DOM0LESS_DOM_LINUX_DEMO_AUTORUN is set
  as "1".
* To disable the autorun, build the stack using the following command:

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

Issue-Id: SCM-4876
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Jiacheng Tang <jiacheng.tang@arm.com>
Change-Id: Iec49632a60cc3fc14ce64c58af066ade9e688cd3