[legal] Add mechanism for explicit per-file licence declarations

For partly historical reasons, various files in the gPXE source tree
are licensed under different, though compatible, terms.  Most of the
code is licensed under GPLv2 with the "or later" clause, but there are
exceptions such as:

  The string.h file, which derives from Linux and is licensed as
  Public Domain.

  The EFI header files, which are taken from the EDK2 source tree and
  are licensed under BSD.

  The 3c90x driver, which has a custom GPL-like licence text.

Introduce a FILE_LICENCE() macro to make licensing more explicit.
This macro should be applied exactly once to each source (.c, .S or
.h) file.  It will cause a corresponding zero-sized common symbol to
be added to any .o files generated from that source file (and hence to
any final gPXE binaries generated from that source file).  Determining
the applicable licences to generated files can then be done using e.g.

  $ objdump -t bin/process.o | grep __licence
  00000000       O *COM*  00000001 .hidden __licence_gpl2_or_later

indicating that bin/process.o is covered entirely by the GPLv2
with the "or later" clause, or

  $ objdump -t bin/rtl8139.dsk.tmp | grep __licence
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_gpl2_only
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_gpl2_or_later
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_public_domain

indicating that bin/rtl8139.dsk includes both code licensed under
GPLv2 (both with and without the "or later" clause) and code licensed
as Public Domain.

Determining the result of licence combinations is currently left as an
exercise for the reader.

src/include/compiler.h

 #define barrier() __asm__ __volatile__ ( "" : : : "memory" )
 #endif /* ASSEMBLY */
 
+/**
+ * @defgroup licences Licence declarations
+ *
+ * For reasons that are partly historical, various different files
+ * within the gPXE codebase have differing licences.
+ *
+ * @{
+ */
+
+/** Declare a file as being in the public domain
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be in the public domain.
+ */
+#define FILE_LICENCE_PUBLIC_DOMAIN \
+	PROVIDE_SYMBOL ( __licence_public_domain )
+
+/** Declare a file as being under version 2 (or later) of the GNU GPL
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under the GNU GPL; "either version 2 of the License, or
+ * (at your option) any later version".
+ */
+#define FILE_LICENCE_GPL2_OR_LATER \
+	PROVIDE_SYMBOL ( __licence_gpl2_or_later )
+
+/** Declare a file as being under version 2 of the GNU GPL
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under version 2 of the GPL, and does not include the
+ * "or, at your option, any later version" clause.
+ */
+#define FILE_LICENCE_GPL2_ONLY \
+	PROVIDE_SYMBOL ( __licence_gpl2_only )
+
+/** Declare a file as being under any version of the GNU GPL
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under the GPL, but does not specify a version.
+ *
+ * According to section 9 of the GPLv2, "If the Program does not
+ * specify a version number of this License, you may choose any
+ * version ever published by the Free Software Foundation".
+ */
+#define FILE_LICENCE_GPL_ANY \
+	PROVIDE_SYMBOL ( __licence_gpl_any )
+
+/** Declare a file as being under the three-clause BSD licence
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under terms allowing redistribution in source and
+ * binary forms (with or without modification) provided that:
+ *
+ *     redistributions of source code retain the copyright notice,
+ *     list of conditions and any attached disclaimers
+ *
+ *     redistributions in binary form reproduce the copyright notice,
+ *     list of conditions and any attached disclaimers in the
+ *     documentation and/or other materials provided with the
+ *     distribution
+ *
+ *     the name of the author is not used to endorse or promote
+ *     products derived from the software without specific prior
+ *     written permission
+ *
+ * It is not necessary for the file to explicitly state that it is
+ * under a "BSD" licence; only that the licensing terms be
+ * functionally equivalent to the standard three-clause BSD licence.
+ */
+#define FILE_LICENCE_BSD3 \
+	PROVIDE_SYMBOL ( __licence_bsd3 )
+
+/** Declare a file as being under the two-clause BSD licence
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under terms allowing redistribution in source and
+ * binary forms (with or without modification) provided that:
+ *
+ *     redistributions of source code retain the copyright notice,
+ *     list of conditions and any attached disclaimers
+ *
+ *     redistributions in binary form reproduce the copyright notice,
+ *     list of conditions and any attached disclaimers in the
+ *     documentation and/or other materials provided with the
+ *     distribution
+ *
+ * It is not necessary for the file to explicitly state that it is
+ * under a "BSD" licence; only that the licensing terms be
+ * functionally equivalent to the standard two-clause BSD licence.
+ */
+#define FILE_LICENCE_BSD2 \
+	PROVIDE_SYMBOL ( __licence_bsd2 )
+
+/** Declare a particular licence as applying to a file */
+#define FILE_LICENCE( _licence ) FILE_LICENCE_ ## _licence
+
+/** @} */
+
+/* This file itself is under GPLv2-or-later */
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <bits/compiler.h>
 
 #endif /* COMPILER_H */