tools: moveconfig: allow to run without any CONFIG specified

I found "tools/moveconfig -s" might be useful for defconfig re-sync.
I could optimize it for re-sync if I wanted, but I do not want to
make the code complex for this feature.

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