waf: allow boards to tweak build

We define a function build() that can be overrode by Board subclasses.

Tools/ardupilotwaf/boards.py

@@ -99,6 +99,9 @@ class Board:
             '-Wl,--gc-sections',
         ]
 
+    def build(self, bld):
+        pass
+
 Board = BoardMeta('Board', Board.__bases__, dict(Board.__dict__))
 
 def get_boards_names():

wscript

@@ -234,6 +234,7 @@ def build(bld):
     _build_dynamic_sources(bld)
 
     bld.add_group('build')
+    boards.get_board(bld.env.BOARD).build(bld)
     _build_common_taskgens(bld)
 
     _build_recursion(bld)