webos-repositories.inc: Add WEBOS_GIT_TAG variable

:Release Notes:
It makes it easier to remove tag param from SRC_URI and more consistent
with branch parameter.

:Detailed Notes:

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Issues Addressed:
[ANG-329] Ongoing metadata improvement/cleanup work

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>

Change-Id: I975e7771a82c65b66d6392eecb52f295a6dc9203