This adds a "waf" buildtype to create-ccan-tree. When passed it
generates a basic wscript for building everything into a libccan.a

It doesn't support running the generator, so you need to provide
config.h
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>