image_types: improve wks path specification

Hardcoding a full input path with zero flexibility goes against everything the
Yocto Project is about. Rework it to let the user specify the wks base
filename with WKS_FILE and it'll search the layers for the wks file and use
it.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>