cmake: improved handling of CONF_FILE cached variable

Fixes: #28134

Minor fix for correctly testing if CONF_FILE variable is defined, and
if it is defined, then a current scope variable will be set with same
name. For a variable that is already present in current scope this has
no effect, but a cached variable (such as one provided with -DCONF_FILE)
will now have both a current scope and cached definition.
This ensures that current scope CONF_FILE will continue to be defined
even when the cached variable is cleared later.

Clearing of the cached variable ensures correct detection if user later
re-invokes CMake with a new -DCONF_FILE value.

The cached version is cleared later in favor of the CACHED_CONF_FILE
that is used between multiple CMake invocations.

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