tools/create-ccan-tree: Add --build-type=TYPE

Replace the --exclude-configurator argument with a
single --build-type=TYPE argument, allowing future expansion of build
system types (and allowing source-tree-only creation). We currently
support:

 * 'make' (just the Makefile, equivalent to --exclude-configurator)
 * 'make+config' (current default)

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>