patman: Convert byte arrays to strings

patman: Convert byte arrays to strings

os.read() returns a byte array in Python 3.5.2 and needs to be converted
into a string. Check if the returned value is an instance of bytes and
if it is decode it as a utf-8 string. If it is not a utf-8 encoded string
the decoding may fail with an exception.

Prior to this fix the comparisions check data == "" would fail when data
was b'' and would cause an infinite memory leaking loop. joins would
also fail with an exception below but due to the infinite loop it never
made it that far.

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