scripts/functions: add a save/restore handler

Saving and restoring the steps requires saving/restoring multiple
directories. Depending on the configuration, some may not exist.

Add a wrapper that checks before creating/extracting the tarballs.

 # m68k specific configuration file

 # depends on EXPERIMENTAL

 config ARCH_m68k

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_CPU

      help

       The m68k architecture