At present all the 'command' methods return bytes. Most of the time we
actually want strings, so change this. We still need to keep the internal
representation as bytes since otherwise unicode strings might break over
a read() boundary (e.g. 4KB), causing errors. But we can convert the end
result to strings.

Add a 'binary' parameter to cover the few cases where bytes are needed.

Signed-off-by: Simon Glass <sjg@chromium.org>