bootutil: PureEdDSA using ED25519

The commit adds support for PureEdDSA, which validates signature
of image rather than hash. This is most secure, available, ED25519
usage in MCUboot, but due to requirement of PureEdDSA to be able
to calculate signature at whole message at once, here image,
it only works on setups where entire image can be mapped to
device address space, so that PSA functions calculating the
signature can see the whole image at once.

The feature is enabled with MCUBOOT_SIGN_PURE MCUboot configuration
option.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>