sysbuild: kconfig: Unset shield config value variable

Fixes an issue with shields that have Kconfig file fragments when
being used with sysbuild, they would be loaded into sysbuild
itself which would then fail because it does not have the Kconfig
tree that zephyr applications have

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 49f9d8e1)