waf: let toolchain loading in charge of the Board class

That allows boards (i.e. subclasses of Board) to use toolchain related
information for configuration. The upcoming PX4 build is an example.

Tools/ardupilotwaf/boards.py

@@ -46,6 +46,9 @@ class Board:
             else:
                 cfg.env[k] = val
 
+        cfg.load('toolchain')
+        cfg.load('compiler_cxx compiler_c')
+
     def configure_env(self, cfg, env):
         # Use a dictionary instead of the convetional list for definitions to
         # make easy to override them. Convert back to list before consumption.

wscript

@@ -78,8 +78,6 @@ def configure(cfg):
     cfg.env.BOARD = cfg.options.board
     boards.get_board(cfg.env.BOARD).configure(cfg)
 
-    cfg.load('toolchain')
-    cfg.load('compiler_cxx compiler_c')
     cfg.load('clang_compilation_database')
     cfg.load('waf_unit_test')
     cfg.load('mavgen')