webos: Add WEBOS_DISTRO_API_VERSION variable

[GF-2615] Invent/create build variable for webos api version

Open-webOS-DCO-1.0-Signed-off-by: Janne Hurskainen <janne.hurskainen@palm.com>

Change-Id: I6ea743f77bdafee1b8d9061a08eeafe2ab110d3f
Reviewed-on: https://g2g.palm.com/1742
Reviewed-by: Build Verification
Reviewed-by: Janne Hurskainen <janne.hurskainen@palm.com>
Tested-by: Janne Hurskainen <janne.hurskainen@palm.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>