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>
