scripts: update west to latest upstream version.

Update to the latest west. This includes a new 'attach' command. There
are also multi-repo commands, but those won't get exposed to the user
unless they install Zephyr using "west init" + "west fetch" (and not,
say, "git clone").

Replace the launchers; they now detect whether zephyr is part of a
multi-repo installation, and run the west code in its own repository
if that is the case.

This also requires an update to:

- the flash/debug CMakeLists.txt, as the new west package is no longer
  executable as a module and must have its main script run by the
  interpreter instead.

- the documentation, to reflect a rename and with a hack to fix
  the automodule directive in flash-debug.rst for now

Signed-off-by: Marti Bolivar <marti@foundries.io>