libfdt: Add fdt_strerror() function to library

This function moves the fdt_strerror() function, currently found in
the test code into the fdt library proper.  This makes life easier for
any library users who want to provide meaningful error messages.  The
function goes into a module of its own, so that users who don't need
the function won't get a copy of it linked in.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

TODO

@@ -1,4 +1,3 @@
-- Move fdt_strerror() to core library for convenience
 - Find node by linux,phandle property
 - Tree traversal functions
 - Graft function