switch from openssl to libmd for sha256

Some versions of openssl are perceived to have a problematic
(non-GPL-compatible) license.  For example, lintian in Debian 11 Bullseye
rejects GPL programs linked with openssl:

    E: mesaflash: possible-gpl-code-linked-with-openssl

This commit switches from using openssl to using libmd for SHA256
computation, which does not have this problem (and which is arguably
preferable for being lighter-weight and more focused than openssl).

Note: a slight difference in the APIs of the two implementations requires
the slightly different arguments to SHA256_Update():

    openssl:

        int SHA256_Update(SHA256_CTX *c, const void *data, size_t len);

    libmd:

        void SHA256_Update(SHA25_CTX *, const uint8_t *, size_t);

This fixes a pedantic lintian message.

Add new commands to man file

Add --sha256-check command, for integrity check bitfiles before writing

These platforms were identified when Debian's buildd tried to build
3.4.0-2 on sid:

    https://buildd.debian.org/status/package.php?p=mesaflash

I used the information on this page to get expected `uname -m` output
for different platforms:

    https://wiki.debian.org/ArchitectureSpecificsMemo

The 3.4.0-2 release was made from commit
5cd5556e of the git repo, with some
debian-local changes.

None of the old releases were uploaded to the official Debian repo,
so list them as UNRELEASED.

3.4.0-1 was the first release uploaded to Debian (unstable), but it was
rejected by the ftpmasters so mark it too as UNRELEASED.

Followup on PR#57 Create readme source

Add --backup-flash and --restore-flash commands

More debs

Add a debian packaging workflow

Just what I found locally in my master branch.

This doesn't work on debian.