webos_cmake: Fix sstate issue in OECMAKE_BUILDPATH

* Juha reported some recipes are not reused from sstate-cache after
  753af77b6dba79825f30bd4b5a4be636b0f19b36, it's caused by vardepvalue
  including whole S value in OECMAKE_BUILDPATH.

* create new OECMAKE_BUILDPATH_SUBDIR variable with vardepvalue and
  prepend it with ${S} in OECMAKE_BUILDPATH

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I22dcb87e586785a77466fd8175bebd4489f7a6b5
Reviewed-on: https://g2g.palm.com/1953
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Juha Kurki <juha.kurki@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>