add $$shadowed() function

add $$shadowed() function

return the build directory corresponding to a given source directory.
this is the identity function if not shadow-building. if input lies
outside the source directory, return empty value.

Change-Id: I2d2a6b1112bd19989fe29cfe19a12d39a0d208c1
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>