manual: add advice about GPL compliance for Buildroot

* +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

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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.