webos_system_bus: Allow alternate locations and naming of sysbus files

:Release Notes:
- Introduce WEBOS_SYSTEM_BUS_FILES_LOCATION, which allows the location
  of the tree of system bus files to install to be changed from the
  default ${S}/service. The installation is skipped if this variable
  is "".

- Handle the case where the files aren't suffixed with .pub and .prv,
  which signifies that the same file is to be used for both public and
  private.

:Detailed Notes:

:Testing Performed:
Removed the explicit installation of the system bus files from
mojolocation-stub.bb and added:

    inherit webos_system_bus
    ...
    WEBOS_SYSTEM_BUS_FILES_LOCATION = "${S}/files/sysbus"

Confirmed that the contents of the packages it built were unchanged.

:QA Notes:

:Issues Addressed:
[GF-9954] webos_system_bus: Allow installation of system bus files from
          ${S}/files/sysbus

Open-webOS-DCO-1.0-Signed-off-by: Herb Kuta <herb.kuta@lge.com>
Change-Id: I9ffa097d126ebda9af242ff3cb397ad4cdd60ba0
Reviewed-on: https://g2g.palm.com/2877
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>