waf: use pre_build hook for ChibiOS

this fixes dynamic uavcan building

Tools/ardupilotwaf/boards.py

@@ -189,6 +189,10 @@ class Board:
         cfg.define('__STDC_FORMAT_MACROS', 1)
 
 
+    def pre_build(self, bld):
+        '''pre-build hook that gets called before dynamic sources'''
+        pass
+
     def build(self, bld):
         bld.ap_version_append_str('GIT_VERSION', bld.git_head_hash(short=True))
         import time
@@ -366,6 +370,14 @@ class chibios(Board):
         super(chibios, self).build(bld)
         bld.load('chibios')
 
+    def pre_build(self, bld):
+        '''pre-build hook that gets called before dynamic sources'''
+        from waflib.Context import load_tool
+        module = load_tool('chibios', [], with_sys_path=True)
+        fun = getattr(module, 'pre_build', None)
+        if fun:
+            fun(bld)
+
 class skyviper_f412(chibios):
     name = 'skyviper-f412'
     def configure_env(self, cfg, env):

Tools/ardupilotwaf/chibios.py

@@ -225,12 +225,13 @@ def configure(cfg):
     if env.HAL_WITH_UAVCAN:
         setup_can_build(cfg)
 
-def build(bld):
+def pre_build(bld):
+    '''pre-build hook to change dynamic sources'''
     load_env_vars(bld.env)
-
     if bld.env.HAL_WITH_UAVCAN:
         bld.get_board().with_uavcan = True
-    
+
+def build(bld):    
     bld(
         # build hwdef.h and apj.prototype from hwdef.dat. This is needed after a waf clean
         source=bld.path.ant_glob('libraries/AP_HAL_ChibiOS/hwdef/%s/hwdef.dat' % bld.env.get_flat('BOARD')),