build: add --no-download option to bootstrap.py

With this option, the script will check that the expected waf version is
present, but will simply fail if it's not, rather than trying to
download it. This allows a package build script to avoid compile-time
network access but still ensure it's using the right waf version.

bootstrap.py

@@ -16,6 +16,10 @@ if os.path.exists("waf"):
         print("Found 'waf', skipping download.")
         sys.exit(0)
 
+if "--no-download" in sys.argv[1:]:
+    print("Did not find {} and no download was requested.".format(WAFRELEASE))
+    sys.exit(1)
+
 try:
     from urllib.request import urlopen, URLError
 except: