webos.inc: Invent WEBOS_DISTRO_PRERELEASE

:Release Notes:
Invent WEBOS_DISTRO_PRERELEASE with default value "devel".

:Detailed Notes:
Recipes which want to use this variable should inherit new
webos_prerelease_dep bbclass.

:Testing Performed:
Tested together with following nyx-modules change which exposes
this value in OSInfo.

:QA Notes:

:Issues Addressed:
[GF-2855]  nyx-modules: Replace NYX_OS_INFO_WEBOS_PRERELEASE with a real
           value
[GF-6757]  Need a flag to indicate production build
[GF-36793] CCC: nyx-modules=91 and WEBOS_DISTRO_PRERELEASE changes

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I8f87c76012cbd27d2d42ebf766ef57fe07303b70
Reviewed-on: https://g2g.palm.com/2557
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>