lib: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

cpukit/libcrypt/crypt-md5.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2003 Poul-Henning Kamp
  * All rights reserved.
  *

cpukit/libcrypt/crypt-sha256.c

@@ -1,4 +1,6 @@
-/*
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2011 The FreeBSD Project. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without

cpukit/libcrypt/crypt-sha512.c

@@ -1,4 +1,6 @@
-/*
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2011 The FreeBSD Project. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without