RFC: bootutil: Add TLV for correct ED25519 usage

[de-nordic]

The PureEdDSA ED25519 signature should be done directly over message, so image in our case. The current implementation is not really PureEdDSA nor HashEdDSA, as we basically pass sha256 as a message to PureEdDSA. Not perfect.

Why is this added? This PR adds TLV that indicates that PureEdDSA has been run over image rather than sha has been calculated and passed to ed25519 for signature/verification.

Problems This only works with devices that are mapping flash to RAM address space, so that image signature verification can directly access whole image as message. This means that configuration with external devices may not work.

Note also that signatures PureEdDSA(image) != PureEdDSA(sha512(image)) != HashEdDSA(sha512(image)), so there is no realu way to use multi-part signature verification with PureEdDSA.

The https://github.com/mcu-tools/mcuboot/pull/2048 is implementation where sha512 is used with PureEdDSA, not perfect.