dtoc: Decode val if it's a byte string

dtoc: Decode val if it's a byte string

With Python 3.5.2 encode will throw an exception if val is a byte array.
Decode it to a string first. This assumes it's utf-8, if it's not valid
utf-8 it will throw an exception.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>