doc: Replace examples of MD5 and SHA1 with SHA256

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>