PR 2115 - Check prefix path write access before starting to build.

Added a check in the options post processing to check is the
prefix path allows writes. No actual write check is made. just
the permissions are checked. If the --no-install options is
used the check is not made.

Moved the --no-install option from the set builder to the options
module.

source-builder/sb/path.py

@@ -89,6 +89,9 @@ def isfile(path):
 def isabspath(path):
     return path[0] == '/'
 
+def iswritable(path):
+    return os.access(host(path), os.W_OK)
+
 def mkdir(path):
     path = host(path)
     if exists(path):