sysbuild: clear local scope EXTRA_CONF_FILE variable

Sysbuild sets the EXTRA_CONF_FILE before loading kconfig.cmake.
This hides any user defined EXTRA_CONF_FILE added to the sysbuild cache.

Clear the local sysbuild EXTRA_CONF_FILE so that the hidden cache
variable re-enters current scope.

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