Please update build-system.rst for commit 859aef026e5

[Markus Armbruster]

docs/devel/build-system.rst claims

    ``config-host.h``, ``$TARGET-NAME/config-target.h``, 
``$TARGET-NAME/config-devices.h``
      These files are used by source code to determine what features
      are enabled.  They are generated from the contents of the corresponding
      ``*.h`` files using the scripts/create_config program. This extracts
      relevant variables and formats them as C preprocessor macros.

scripts/create_config went away in your commit 859aef026e "meson:
replace create-config with meson configure_file", 2020-08-21.  Please
update build-system.rst.