webos_cmake: Add weak assignments for WEBOS_TARGET_*
webos_cmake: Add weak assignments for WEBOS_TARGET_*

* WEBOS_TARGET_MACHINE_IMPL, WEBOS_TARGET_CORE_OS are
  defined only in webos.inc, building for different DISTRO than webos
  fails to parse all recipes which are inheriting webos_cmake

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I0012c83184b29fa59c4fb6855b40a5bcb4a22f24
Reviewed-on: https://g2g.palm.com/1852
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@lge.com>
Tested-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@lge.com>
Reviewed-by: Susan Montooth <be0b8b1fe2cc6a744be9b1ae6516e077aa323e09@lge.com>

openwebos/meta-webos,openwebos/meta-webos,nizovn/meta-webos-ports,Garfonso/meta-webos-ports,openwebos/meta-webos,Garfonso/meta-webos-ports,openwebos/meta-webos,nizovn/meta-webos-ports