manual: add advice about GPL compliance for Buildroot

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/using.txt

@@ -284,3 +284,30 @@ Buildroot, with the name used in the manifest file:
 * +BSD-2c+: BSD 2-clause license;
 * +PROPRIETARY+: marks a non-opensource package;
   Buildroot does not save any licensing info or source code for these packages.
+
+Complying with the Buildroot license
+------------------------------------
+
+Buildroot itself is an opensource software, released under the
+http://www.gnu.org/licenses/old-licenses/gpl-2.0.html[GNU General Public
+License, version 2] or (at your option) any later version.
+However, being a build system, it is not normally part of the end product:
+if you develop the root filesystem, kernel, bootloader or toolchain for a
+device, the code of Buildroot is only present on the development machine, not
+in the device storage.
+
+Nevertheless, the general view of the Buildroot developers is that you should
+release the Buildroot source code along with the source code of other packages
+when releasing a product that contains GPL-licensed software.
+This is because the
+http://www.gnu.org/licenses/old-licenses/gpl-2.0.html[GNU GPL]
+defines the "'complete source code'" for an executable work as "'all the
+source code for all modules it contains, plus any associated interface
+definition files, plus the scripts used to control compilation and installation
+of the executable'".
+Buildroot is part of the 'scripts used to control compilation and
+installation of the executable', and as such it is considered part of the
+material that must be redistributed.
+
+Keep in mind this is only the Buildroot developers' opinion, and you should
+consult your legal department or lawyer in case of any doubt.