imgtool: Add pure signature support

Adds PureEdDSA signature support.

The change includes implementation of SIG_PURE TLV that, when present,
indicates the signature that is present is Pure type.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Mateusz Michalek <mateusz.michalek@nordicsemi.no>