kconfiglib: Save previous configuration to .config.old

Update Kconfiglib (and menuconfig, just to sync) to upstream revision
094f4a9622046, to add the commit below.

  Save existing configuration to .<filename>.old in write_config()

  Add a default-True 'save_old' flag to write_config(). If 'save_old' is
  True and an existing configuration file is being overwritten, a copy
  of the old configuration file is saved to .<filename>.old (e.g.
  .config.old) in the same directory.

  Errors are ignored, as the old configuration would usually just be a
  nice-to-have, and not essential.

  The same functionality could be added for minimal configuration files
  and headers, but it's probably most useful for configuration files.

Other changes:

  - Parsing performance is improved a bit

  - scripts/kconfig/kconfig.py now prints the path to the merged
    configuration in zephyr/.config, to make it a bit easier to
    discover.

Fixes: #2907

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>