lib: add crypt subsystem

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: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>

include/crypt.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/* Copyright (C) 2020 Steffen Jaeckel <jaeckel-floss@eyet-services.de> */
+
+/**
+ * Compare should with the processed passphrase.
+ *
+ * @should      The crypt-style string to compare against
+ * @passphrase  The plaintext passphrase
+ * @equal       Pointer to an int where the result is stored
+ *                 '0' = unequal
+ *                 '1' = equal
+ */
+void crypt_compare(const char *should, const char *passphrase, int *equal);