imgtool: Added support for providing the signature by 3rd party

The sign command was extended so it now allow to provide the signature
as base64 formatted RAW file using --fix-sig along with the relevant
public key --fix-sig-pubkey.

This patch is added for support the case where the party which produces
the image dose not have access to the signing image key but must request
third party for the signature.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>