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>
diff --git a/lib/Kconfig b/lib/Kconfig
index ad0cd52..ad4d75e 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -297,6 +297,7 @@
 
 source lib/rsa/Kconfig
 source lib/crypto/Kconfig
+source lib/crypt/Kconfig
 
 config TPM
 	bool "Trusted Platform Module (TPM) Support"