lsb: Add weak assignment to WEBOS_TARGET_CORE_OS

* this whole .bbappend should be in separate distro layer
* WEBOS_TARGET_CORE_OS is defined only in webos.inc, lsb bbappend
  fails to parse when building for different DISTRO than webos

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I8f74c3ac6dc3f3b4a7a2da3cf4a323fc0f032ca5
Reviewed-on: https://g2g.palm.com/1851
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>