dtc/libfdt: Add README clarifying licensing

The fact that the dtc and libfdt are distributed together, but have
different licenses, can be a bit confusing.  Several people have
enquired as to what the deal is with the libfdt licensing, so this
patch adds a README clarifying the situation with a rationale.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Jon Loeliger <jdl@jdl.com>

README.license

@@ -0,0 +1,56 @@
+Licensing and contribution policy of dtc and libfdt
+===================================================
+
+This dtc package contains two pieces of software: dtc itself, and
+libfdt which comprises the files in the libfdt/ subdirectory.  These
+two pieces of software, although closely related, are quite distinct.
+dtc does not incoporate or rely on libfdt for its operation, nor vice
+versa.  It is important that these two pieces of software have
+different license conditions.
+
+As the copyright banners in each source file attest, dtc is licensed
+under the GNU GPL.  The full text of the GPL can be found in the file
+entitled 'GPL' which should be included in this package.  dtc code,
+therefore, may not be incorporated into works which do not have a GPL
+compatible license.
+
+libfdt, however, is GPL/BSD dual-licensed.  That is, it may be used
+either under the terms of the GPL, or under the terms of the 2-clause
+BSD license (aka the ISC license).  The full terms of that license are
+given in the copyright banners of each of the libfdt source files.
+This is, in practice, equivalent to being BSD licensed, since the
+terms of the BSD license are strictly more permissive than the GPL.
+
+I made the decision to license libfdt in this way because I want to
+encourage widespread and correct usage of flattened device trees,
+including by proprietary or otherwise GPL-incompatible firmware or
+tools.  Allowing libfdt to be used under the terms of the BSD license
+makes that it easier for vendors or authors of such software to do so.
+
+This does mean that libfdt code could be "stolen" - say, included in a
+proprietary fimware and extended without contributing those extensions
+back to the libfdt mainline.  While I hope that doesn't happen, I
+believe the goal of allowing libfdt to be widely used is more
+important than avoiding that.  libfdt is quite small, and hardly
+rocket science; so the incentive for such impolite behaviour is small,
+and the inconvenience caused therby is not dire.
+
+Licenses such as the LGPL which would allow code to be used in non-GPL
+software, but also require contributions to be returned were
+considered.  However, libfdt is designed to be used in firmwares and
+other environments with unusual technical constraints.  It's difficult
+to anticipate all possible changes which might be needed to meld
+libfdt into such environments and so difficult to suitably word a
+license that puts the boundary between what is and isn't permitted in
+the intended place.  Again, I judged encouraging widespread use of
+libfdt by keeping the license terms simple and familiar to be the more
+important goal.
+
+**IMPORTANT** It's intended that all of libfdt as released remain
+permissively licensed this way.  Therefore only contributions which
+are released under these terms can be merged into the libfdt mainline.
+
+
+David Gibson <david@gibson.dropbear.id.au>
+(principal original author of dtc and libfdt)
+2 November 2007