webos.inc: Add build timestamp to build-id.txt

:Release Notes:
Add build DATETIME to build-id.txt, for use by pruning script
and other utilities.

:Detailed Notes:

:Testing Performed:
Examined output in build-id.txt

:QA Notes:
Only a build-system change with no runtime effect.

:Issues Addressed:
[ES-998] Add build timestamp to build-id.txt

Open-webOS-DCO-1.0-Signed-off-by: Susan Montooth <susan.montooth@lge.com>

Change-Id: I71ee5c8e7455baaf607dec1018a9387d23a482a3
Reviewed-on: https://g2g.palm.com/6012
Reviewed-by: DCO Verification
Build: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>