cmake: improved variable handling in zephyr_check_cache()

A cache variable specified by the user as `-DVAR=value` will have the
type UNINITIALIZED in the CMake cache.

Later CMake re-runs will execute the line:
set(${variable} ${CACHED_${variable}} CACHE STRING
    "Selected ${variable_text}" FORCE)

This will update the cache type from UNINITIALIZED to STRING and thus
may be picked up by other tools as a change.

To ensure the type is initialized to string, this is added:
set(${variable} ${${variable}} CACHE STRING "Selected ${variable_text}")

This code will not change the value of an existing CMake cache variable
because there is no FORCE, but what it will do is to update the cache
variable type to STRING.

As part of this, include the existing parent scope setting of variable
inside the `if(DEFINED ...)` as there is no reason to clear a variable
there is not having a value.

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