Add libminshacrypt static lib

This is a sha-only version of libmincrypt, to provide the new
SHA_hash implementation to binaries that require libcrypto_static
Add support for ECDSA P-256 with SHA256

This adds a small EC library that is capable of verifying a signature of
SHA256 with ECDSA on the NIST P-256 curve.

Change-Id: I2a16639c92a77e8e4783c47ffbc56676de56eb59