Licenses/README: Update some style and add explicit license to the document

- Add an SPDX license tag to the file, saying it's GPL-2.0.
- From the Linux Kernel v4.17-rc4, import the "License identifier
  syntax" section as-is from Documentation/process/license-rules.rst
  and then change it to be clearer about examples from the Linux Kernel
  vs examples found in U-Boot, and when we're talking about U-Boot.

Signed-off-by: Tom Rini <trini@konsulko.com>

Licenses/README

@@ -1,3 +1,5 @@
+SPDX-License-Identifier: GPL-2.0
+
   U-Boot is Free Software.  It is copyrighted by Wolfgang Denk and
 many others who contributed code (see the actual source code and the
 git commit messages for details).  You can redistribute U-Boot and/or
@@ -31,27 +33,107 @@ information, ...) which makes automatic processing a nightmare.
 
 To make this easier, such license headers in the source files will be
 replaced with a single line reference to Unique License Identifiers
-as defined by the Linux Foundation's SPDX project [1].  For example,
-in a source file the full "GPL v2.0 or later" header text will be
-replaced by a single line:
-
-	SPDX-License-Identifier:	GPL-2.0+
-
-Ideally, the license terms of all files in the source tree should be
-defined by such License Identifiers; in no case a file can contain
-more than one such License Identifier list.
+as defined by the Linux Foundation's SPDX project [1].
 
 If a "SPDX-License-Identifier:" line references more than one Unique
 License Identifier, then this means that the respective file can be
 used under the terms of either of these licenses, i. e. with
 
-	SPDX-License-Identifier:	GPL-2.0+	BSD-3-Clause
+	SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause
 
 you can choose between GPL-2.0+ and BSD-3-Clause licensing.
 
 We use the SPDX Unique License Identifiers here; these are available
 at [2].
 
+License identifier syntax
+-------------------------
+
+1. Placement:
+
+   The SPDX license identifier in U-Boot files shall be added at the first
+   possible line in a file which can contain a comment.  For the majority
+   or files this is the first line, except for scripts which require the
+   '#!PATH_TO_INTERPRETER' in the first line.  For those scripts the SPDX
+   identifier goes into the second line.
+
+|
+
+2. Style:
+
+   The SPDX license identifier is added in form of a comment.  The comment
+   style depends on the file type::
+
+      C source:	// SPDX-License-Identifier: <SPDX License Expression>
+      C header:	/* SPDX-License-Identifier: <SPDX License Expression> */
+      ASM:	/* SPDX-License-Identifier: <SPDX License Expression> */
+      scripts:	# SPDX-License-Identifier: <SPDX License Expression>
+      .rst:	.. SPDX-License-Identifier: <SPDX License Expression>
+      .dts{i}:	// SPDX-License-Identifier: <SPDX License Expression>
+
+   If a specific tool cannot handle the standard comment style, then the
+   appropriate comment mechanism which the tool accepts shall be used. This
+   is the reason for having the "/\* \*/" style comment in C header
+   files. There was build breakage observed with generated .lds files where
+   'ld' failed to parse the C++ comment. This has been fixed by now, but
+   there are still older assembler tools which cannot handle C++ style
+   comments.
+
+|
+
+3. Syntax:
+
+   A <SPDX License Expression> is either an SPDX short form license
+   identifier found on the SPDX License List, or the combination of two
+   SPDX short form license identifiers separated by "WITH" when a license
+   exception applies. When multiple licenses apply, an expression consists
+   of keywords "AND", "OR" separating sub-expressions and surrounded by
+   "(", ")" .
+
+   License identifiers for licenses like [L]GPL with the 'or later' option
+   are constructed by using a "+" for indicating the 'or later' option.::
+
+      // SPDX-License-Identifier: GPL-2.0+
+      // SPDX-License-Identifier: LGPL-2.1+
+
+   WITH should be used when there is a modifier to a license needed.
+   For example, the linux kernel UAPI files use the expression::
+
+      // SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note
+      // SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note
+
+   Other examples using WITH exceptions found in the linux kernel are::
+
+      // SPDX-License-Identifier: GPL-2.0 WITH mif-exception
+      // SPDX-License-Identifier: GPL-2.0+ WITH GCC-exception-2.0
+
+   Exceptions can only be used with particular License identifiers. The
+   valid License identifiers are listed in the tags of the exception text
+   file.
+
+   OR should be used if the file is dual licensed and only one license is
+   to be selected.  For example, some dtsi files are available under dual
+   licenses::
+
+      // SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+
+   Examples from U-Boot for license expressions in dual licensed files::
+
+      // SPDX-License-Identifier: GPL-2.0 OR MIT
+      // SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause
+
+   AND should be used if the file has multiple licenses whose terms all
+   apply to use the file. For example, if code is inherited from another
+   project and permission has been given to put it in U-Boot, but the
+   original license terms need to remain in effect::
+
+      // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) AND MIT
+
+   Another other example where both sets of license terms need to be
+   adhered to is::
+
+      // SPDX-License-Identifier: GPL-1.0+ AND LGPL-2.1+
+
 [1] http://spdx.org/
 [2] http://spdx.org/licenses/