kconfig: Make nconf-cfg.sh executable

Although it's not required for the build *conf-cfg.sh scripts to be
executable (they're run by CONFIG_SHELL), let's be consistent with other
scripts.
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>