Do not require system bus files to be configured

If a JSON system bus file does not contain any paths, it is an unnatural
act to rename them as .in files.

- Modify webos_build_system_bus_files() to install files with extensions
  of ".pub" or ".prv" without being configured.
- Modify REFERENCE.md to reflect the new behavior.

[GF-1749] Modify webos_build_system_bus_files() to install unconfigured files

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>
Change-Id: If8c2d72e3a92233c45bf3db7198ff3ca7908962c