Add the basic functionality required to support the standard crypt
format.
The files crypt-sha256.c and crypt-sha512.c originate from libxcrypt and
their formatting is therefor retained.
The integration is done via a crypt_compare() function in crypt.c.

```
libxcrypt $ git describe --long --always --all
tags/v4.4.17-0-g6b110bc
```
Signed-off-by: NSteffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NHeiko Schocher <hs@denx.de>