imgtool: fix encrypting hex images

Fixes padding hex images when encrypting. The issues stems from binaries
using `bytes` and IntelHex returning `array` where `bytes` cannot be
appended to, so use `.extend()` instead.

Signed-off-by: Fabio Utzig <utzig@apache.org>
Signed-off-by: David Brown <david.brown@linaro.org>