west: zcmake.py: support UNINITIALIZED type if CMakeCache.txt file

A variable set by the user with `-DVAR=<val>` will be given the type
UNINITIALIZED.

This results in the variable not being read into the cmake_cache.

Support reading of CMake cache variables of type UNINITIALIZED.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>