mgmt/imgmgr: Acccept shorter values of data sha
mgmt/imgmgr: Acccept shorter values of data sha

We will accept SHA trimmed to any length in "sha" field. This is to
allow shorter hashes (as we do not really need 128 bit for upload
resume...) and let client decide what length is good enough to reliably
identify upload to resume.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core