Mbed-TLS is a tiny TLS implementation designed for embedded environment which
can greatly reduce the disk space requirement compared to OpenSSL. While we
already have crypto_kernel for this purpose and Mbed-TLS lacking hash/cipher
support can cause reduced functionality, there're situations where AF_ALG is
not available but we're fine with limited scenarios like LUKS2 only.
These are configurable in build time, to force default backward compatibility use
--with-plain-hash=ripemd160 --with-plain-cipher=aes --with-plain-mode=cbc-essiv:sha256
configure options.
Fixes#758.
