sllin: switch to updated upstream

Changes:
- Switch the SRC_URI of the sllin driver recipe to point at the new
  revived upstream location, update to their latest commit to get
  newer kernel support, and drop all the now unnecessary local
  patches.
- Added a patch to fix compilation against 5.4 kernels before
  5.4.110.  This will be worked with upstream as time permits.
- Rename the sllin driver recipe with a _git suffix to match OE
  style expectations.
- Added the sllin driver to packagegroup-agl-demo as was being done
  in previous releases.
- Split the demo script + service unit and lin_config configuration
  out of the sllin driver recipe into a new sllin-demo recipe so
  that they're no longer conflated.  This should make it easier for
  others to reuse the sllin driver recipe.
- Update the SRC_URI and SRCREV of the lin_config recipe to also
  point at the new upstream location/version since it comes out of
  the same repository.
- Add the new master mode "-m" flag to the lin_config command line
  in the start_lin_demo.sh script (now in sllin-demo).  This turns
  out to be required for our demo usecase as upstream seem to have
  inadvertently changed the default behavior.

Bug-AGL: SPEC-4404

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: If42a825e65d93b3fa11cfddd5d6b33ec410f7cc4