Add can-dev-mapping-helper

Add can-dev-mapping-helper

Changes:
- Add a can-dev-mapping-helper.sh script and systemd unit to drive
  it, the script checks if the CAN devices specified in the low-can
  configuration file /etc/dev-mapping.conf exist, and if not brings
  up corresponding virtual CAN devices.
- Add a systemd over-ride conf file for agl-service-can-low-level
  that makes can-dev-mapping-helper a dependency. This should ensure
  that the low-can binding always starts even if there are no physical
  CAN hardware devices.
- Added a Before statement to sllin-demo.service to ensure that it
  will run before can-dev-mapping-helper and ensure that lin_config
  can create sllin0 correctly.
- Removed the sllin-demo-virtual systemd unit, as it will no longer
  be required with can-dev-mapping-helper.
- Updated the /var/run paths in sllin-demo.service and associated
  start_lin_demo.sh script to quiet systemd warnings about /var/run
  usage.

Bug-AGL: SPEC-3540

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ife02f75f57ac7bbdee6b02217e2ab657fe8c36e0