tools: moveconfig: compute file paths just once

The paths to .config, include/autoconf.mk, include/config/auto.conf
are not changed during the defconfig walk.  Compute them only once
when a new class instance is created.

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