mincrypt: merge the two RSA verifiers

The e=3 and e=65537 implementations are nearly identical, refactor
them down into one.

(cherry picked from commit 8fdbf97652b76b19b4faff522ddcf4a9da543a86)

Change-Id: I9f858204d52a0ae880170d955bf8f29fed57ee94
6 files changed