dtoc: Decode strings for struct.unpack on python 3.x

On python 3.x struct.unpack will complain if we provide it with a
string since it expects to operate on a bytes object. In order to
satisfy this requirement, encode the string to a bytes object when
running on python 3.x.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Simon Glass <sjg@chromium.org>