waf: create program_dir directory

There's an upcoming patch that moves all static libraries to
`build/<board>/lib/`. That way, the program directory won't be created
automatically by the build system and that will cause problems for PX4 builds,
since it builds ardupilot programs as static libraries and copies stuff to the
program directory.

Tools/ardupilotwaf/ardupilotwaf.py

@@ -5,6 +5,7 @@ from __future__ import print_function
 from waflib import Logs, Options, Utils
 from waflib.Build import BuildContext
 from waflib.Configure import conf
+from waflib.TaskGen import before_method, feature
 import os.path, os
 from collections import OrderedDict
 
@@ -186,6 +187,17 @@ def ap_stlib(bld, **kw):
 
     bld.stlib(**kw)
 
+_created_program_dirs = set()
+@feature('cxxstlib', 'cxxprogram')
+@before_method('process_rule')
+def ap_create_program_dir(self):
+    if not hasattr(self, 'program_dir'):
+        return
+    if self.program_dir in _created_program_dirs:
+        return
+    self.bld.bldnode.make_node(self.program_dir).mkdir()
+    _created_program_dirs.add(self.program_dir)
+
 @conf
 def ap_find_tests(bld, use=[]):
     if not bld.env.HAS_GTEST: