tools: moveconfig: skip savedefconfig if .config was not updated

If no CONFIG option is moved to the .config, no need to sync the
defconfig file.  This accelerates the processing by skipping
needless "make savedefconfig".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>