bootutil: Add SIG_PURE TLV

The commit adds SIG_PURE TLV that should be used as TLV indicating
that the signature attached to image has been calculated over
entire image, rather than digest of image.

This is generic flag as the "pure" usage may be applied to, potentially,
any signature algorithm.

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