waf: cxx_checks: define ap_common_checks() instead of configure()

That allow loading the cxx_checks tool before the calls to configure_env().
That way, we can add check functions to be used across boards.

Tools/ardupilotwaf/boards.py

@@ -31,6 +31,7 @@ class Board:
     def configure(self, cfg):
         cfg.env.TOOLCHAIN = self.toolchain
         cfg.load('toolchain')
+        cfg.load('cxx_checks')
 
         env = waflib.ConfigSet.ConfigSet()
         self.configure_env(cfg, env)
@@ -52,7 +53,7 @@ class Board:
             else:
                 cfg.env[k] = val
 
-        cfg.load('cxx_checks')
+        cfg.ap_common_checks()
 
     def configure_env(self, cfg, env):
         # Use a dictionary instead of the convetional list for definitions to

Tools/ardupilotwaf/cxx_checks.py

@@ -24,7 +24,10 @@ Example::
         cfg.load('cxx_checks')
 """
 
-def configure(cfg):
+from waflib.Configure import conf
+
+@conf
+def ap_common_checks(cfg):
     cfg.check(
         compiler='cxx',
         fragment='''