waf: add command 'list_boards'

The command will be used to allow travis script to know which boards are
already supported by the waf build system. This is useful while the transition
to the new build system is on going. The command can be removed when the
transition is done and it's not used anywhere besides travis script.

wscript

@@ -1,6 +1,8 @@
 #!/usr/bin/env python
 # encoding: utf-8
 
+from __future__ import print_function
+
 import os.path
 import sys
 sys.path.insert(0, 'Tools/ardupilotwaf/')
@@ -208,6 +210,9 @@ def collect_dirs_to_recurse(bld, globs, **kw):
             dirs.append(d.parent.relpath())
     return dirs
 
+def list_boards(ctx):
+    print(*BOARDS_NAMES)
+
 def build(bld):
     # NOTE: Static library with vehicle set to UNKNOWN, shared by all
     # the tools and examples. This is the first step until the