Extend sign/create command so it allows to export image's byte string
which is the substrate for the image signature. The new option is
'--vector-to-sign'. It might takes 'payload' or 'digest'.
The exported data might be used to calculate the signature externally.
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
dfce0be6a1