extmod/crypto-algorithms: Add source to header and populate copyright.

As per the README.md of the upstream source at
https://github.com/B-Con/crypto-algorithms, this source code was released
into the public domain, so make that explicit in the copyright line in the
header.

extmod/crypto-algorithms/sha256.c

@@ -1,7 +1,8 @@
 /*********************************************************************
+* Source:     https://github.com/B-Con/crypto-algorithms
 * Filename:   sha256.c
 * Author:     Brad Conte (brad AT bradconte.com)
-* Copyright:
+* Copyright:  This code is released into the public domain.
 * Disclaimer: This code is presented "as is" without any guarantees.
 * Details:    Implementation of the SHA-256 hashing algorithm.
               SHA-256 is one of the three algorithms in the SHA2

extmod/crypto-algorithms/sha256.h

@@ -1,7 +1,8 @@
 /*********************************************************************
+* Source:     https://github.com/B-Con/crypto-algorithms
 * Filename:   sha256.h
 * Author:     Brad Conte (brad AT bradconte.com)
-* Copyright:
+* Copyright:  This code is released into the public domain.
 * Disclaimer: This code is presented "as is" without any guarantees.
 * Details:    Defines the API for the corresponding SHA1 implementation.
 *********************************************************************/