Both SHA1 and (especially) MD5 are no longer as safe as they once were for
cryptographic use. Replaces examples which use them with examples using
SHA256 instead. This will provide more-secure defaults for users who use
documentation examples as a base for their own use. This is not too
necessary for non-verified-boot scenarios (since someone could just replace
the checksum), but I wanted to be complete.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

Move this documentation over to reST. Move the example files into a files/
directory so they are still separate.

Do a few minor updates while we are here:
- Tidy up sandbox build instructions
- Update my github account name
- Add some talks and links

Signed-off-by: Simon Glass <sjg@chromium.org>