waf: fix programs searches

Here we do the following:
    1) Move the pkg-config check to boards.py so we only check of it for
       linux boards
    2) Use cfg.find_toolchain_program() if possible, for example, for AR

Tools/ardupilotwaf/boards.py

@@ -218,6 +218,8 @@ class linux(Board):
     def configure_env(self, cfg, env):
         super(linux, self).configure_env(cfg, env)
 
+        cfg.find_toolchain_program('pkg-config', var='PKGCONFIG')
+
         env.DEFINES.update(
             CONFIG_HAL_BOARD = 'HAL_BOARD_LINUX',
             CONFIG_HAL_BOARD_SUBTYPE = 'HAL_BOARD_SUBTYPE_LINUX_NONE',

Tools/ardupilotwaf/toolchain.py

@@ -133,8 +133,8 @@ def configure(cfg):
     _filter_supported_c_compilers('gcc', 'clang')
     _filter_supported_cxx_compilers('g++', 'clang++')
 
-    cfg.env.AR = cfg.env.TOOLCHAIN + '-ar'
-    cfg.env.PKGCONFIG = cfg.env.TOOLCHAIN + '-pkg-config'
+    cfg.find_toolchain_program('ar')
+
     cfg.msg('Using toolchain', cfg.env.TOOLCHAIN)
     cfg.load('compiler_cxx compiler_c')