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
The e=3 and e=65537 implementations are nearly identical, refactor
them down into one.
(cherry picked from commit 8fdbf97652)
Change-Id: I9f858204d52a0ae880170d955bf8f29fed57ee94