Rework J1939 and ISO TP feature enabling

Rework J1939 and ISO TP feature enabling

Rework the J1939 and ISO TP feature enabling logic in config.cmake to:
- define WITH_FEATURE_J1939 and WITH_FEATURE_ISOTP to default to off,
  thus requiring explicit enabling.  This is safer than making
  assumptions based solely on the presence of the headers at build
  time.
- honor WITH_FEATURE_* definitions given on the cmake command-line;
  the checks for header presence only over-ride an "ON" value if the
  required header is not present.

Bug-AGL: SPEC-3538

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ie2b84ac50bbed0bbb775cae49f7f4fbe14f2f8ae