imgtool: Add support for calculating SHA512

The adds support for hashing image with SHA512, to allow
SHA512-ED25519-SHA512 signature.

To support above --sha parameter has been added that can take value:
 auto, 256, 384, 512
to select sha, where auto brings the default behaviour, or current,
behaviour. The sha provided here is tested against key so not all
combinations are supported.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>