waf: allow to run wrapper on windows

On Windows we have 2 issues with the current wrapper:
	1. We can't call the binary directly relying on shebang. Now we
	   call python and pass the script as an argument
	2. Use os.path.join() with all components to derive the right
	   waf-light location

waf

@@ -1,21 +1,25 @@
 #!/usr/bin/python
 
 from __future__ import print_function
-import os
+import subprocess
 import os.path as p
 import sys
 
 d = p.dirname(p.realpath(__file__))
+waf_light = p.join(d, 'modules', 'waf', 'waf-light')
+
 try:
-    os.execv(os.path.join(d, 'modules/waf/waf-light'), sys.argv)
-except OSError:
-    print('Missing waf submodule. Trying to get it')
+    subprocess.check_call(['python', waf_light] + sys.argv[1:])
+except subprocess.CalledProcessError as e:
+    if e.returncode != 2 or p.isfile(waf_light):
+        raise e
 
-    from subprocess import check_call, CalledProcessError
+    print('Missing waf submodule. Trying to get it')
 
     try:
-        check_call(['git', 'submodule', 'update', '--init', 'modules/waf'])
-    except CalledProcessError:
+        subprocess.check_call(['git', 'submodule', 'update', '--init',
+                               'modules/waf'])
+    except subprocess.CalledProcessError:
         print('Could not update submodule', file=sys.stderr)
         sys.exit(1)