imgtool: Add --pure switch to allow calculating signatures over entire images

[de-nordic]

The PR adds --pure switch to imgtool that allows to run ED25519 over entire image calculating the PureEdDSA signature.

The imgtool will add SIG_PURE TLV as indicator that whatever signature is used is calculated directly over image. The PR also provides different output for signature verification, where there is signature reported on verification, rather than digest ~~as digest is no longer available~~, but will provide digest if key is not provided during verification.

Depends on https://github.com/mcu-tools/mcuboot/pull/2029 -- that reserves the SIG_PURE in MCUboot code Based on https://github.com/mcu-tools/mcuboot/pull/2048

[de-nordic]

May be interesting for you @taltenbach

[nvlsianpu]

@de-nordic I would merge that. Any obstacles?

[nvlsianpu]

@de-nordic Should we proceed with this patch?

[nordicjm]

@d3zd3z can you review?