webos: Add WEBOS_DISTRO_RELEASE_CODENAME variable

[GF-2614] Invent/create build variable for release codename

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

Change-Id: I007f4371a987d0bf639f4ef4792c572b70ca5f6d
Reviewed-on: https://g2g.palm.com/1751
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>