bvb_refimpl: Hide SHA and RSA code from application code.

This hides the code implementing the few RSA and SHA bits we're using
from the application, enabling us to change to another crypto
implementation in the future if we so desire.

Since we have limited control over the build system used, this is
best-effort by playing C preprocessor games. The approach used is
similar to how other C libraries do this (GLib, GTK+, D-Bus).

TEST=Manually tested.

Change-Id: Icd3092d44d1b38419b657839c39f27b051bdcebe
15 files changed