Log Message:
Update to 0.16:

* Documentation changes to match Buildtool 0.16.

  - Track the renaming of defines, which now include the language name in
    them.

  - Added information about the bt_check_prog_make function.

  - Documented the new 'test' target/stage.

  - Documented the new BT_IS_TEST definition in the 'program' type.

  - Documented the new 'bt_logic.conf' system-wide configuration file.

  - Documented the new '--destdir' flag in bt_logic.

  - Documented the new '--workdir' flag in bt_config and bt_logic, as well
    as the BT_USE_WORKDIR variable, related to it.

  - Documented the new bt_echo and bt_source functions in bt_swcgen.

  - Rewritten the bt_swcgen chapter to match reality (no more M4).  This
    should have happened in the previous version.

* Dropped some leftovers of documentation about the bt_make module and
  related Makefiles.  The information has been reworded to match logic
  scripts where appropiate.

* Fixed the description of bt_define and bt_define_unquoted functions, as
  they take a pair of values, not a list of variables.