waf: toolchain: clang: use waf to find toolchain path

The find_realexec_path function was used for finding the toolchain path mostly
because of two reasons:

 1) We couldn't really use CXX or CC variables because the user could set those
 from the OS's environment and Waf wouldn't look for the executable file in
 that case.

 2) Our CI configuration sets up symlinks for ccache and find_realexec_path
 works around that issue.

The bad side about using find_realexec_path() is that, besides working aroung
symlinks, it does the same thing that is done by Waf. This patch removes the
dependency for such a function by addressing each of the reasons above stated:

 1) We create a local copy of os.environ and, if there's a variable with the
 same name we are using, we remove it from the local copy.

 2) As done before, we are looking for the cross ar program instead of gcc
 program, since that is not used for ccache symlinks.

Tools/ardupilotwaf/toolchain.py

@@ -17,30 +17,6 @@ from waflib.Tools import clang, clangxx, gcc, gxx
 
 import os
 
-def find_realexec_path(cfg, filename, path_list=[]):
-    if not filename:
-        return ''
-
-    if not path_list:
-        path_list = cfg.environ.get('PATH','').split(os.pathsep)
-
-    for dir in path_list:
-        path = os.path.abspath(os.path.expanduser(os.path.join(dir, filename)))
-
-        if os.path.isfile(path):
-            if os.path.islink(path):
-                realpath = os.path.realpath(path)
-
-                if filename not in os.path.basename(realpath):
-                    continue
-                else:
-                    return os.path.dirname(realpath)
-
-            else:
-                return os.path.dirname(path)
-
-    cfg.fatal('Could not find real exec path to %s in path_list %s:' % (filename, path_list))
-
 def _set_toolchain_prefix_wrapper(tool_module, var, compiler_names):
     original_configure = tool_module.configure
     def new_configure(cfg):
@@ -73,10 +49,22 @@ def _clang_cross_support(cfg):
     prefix = cfg.env.TOOLCHAIN + '-'
 
     cfg.find_program(prefix + 'gcc', var='CROSS_GCC')
-    toolchain_path = os.path.join(find_realexec_path(cfg, prefix + 'ar'),
-                                  '..')
-    toolchain_path = os.path.abspath(toolchain_path)
 
+    environ = dict(os.environ)
+    if 'TOOLCHAIN_CROSS_AR' in environ:
+        # avoid OS's environment to mess up toolchain path finding
+        del environ['TOOLCHAIN_CROSS_AR']
+    try:
+        cfg.find_program(
+            prefix + 'ar',
+            var='TOOLCHAIN_CROSS_AR',
+            environ=environ,
+        )
+    except Errors.ConfigurationError as e:
+        cfg.fatal('toolchain: clang: couldn\'t find toolchain path', ex=e)
+
+    toolchain_path = os.path.join(cfg.env.TOOLCHAIN_CROSS_AR[0], '..', '..')
+    toolchain_path = os.path.abspath(toolchain_path)
     cfg.msg('Using toolchain path for clang', toolchain_path)
 
     sysroot = cfg.cmd_and_log(