Boot: Add byte cast when signing payload

This commit adds an explicit cast to `bytes` for the payload
when signing data with imgtool. On some systems, the data may be
provided as a `ByteArray`, which will cause the signing function
to fail since the underlying library expects `bytes`.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
Change-Id: I7405d9e76570d61cc745dcd81e6033dec637d2f3