legal-info: infrastructure to collect legally-relevant material

legal-info: infrastructure to collect legally-relevant material

This allows to automatically collect material that may be needed to comply with
the license of packages that Buildroot prepares for the target device.

The core of the implementation is made by the following parts:
 - in package/pkg-utils.mk some helper functions are defined for common actions
   such as generating a warning, producing info about a package etc;
 - in package/pkg-gentargets.mk, within the GENTARGETS framework, a new
   <PKG>-legal-info target produces all the info for a given package;
 - Makefile implements the top-level targets:
   - legal-info-prepare creates the output directory and produces legal info
     about Buildroot itself and the toolchain, which mostly means just warning
     the user that this is not implemented;
   - legal-info, the only target that is supposed to be used directly, depends
     on all of the above and finishes things by producing the README files from
     the various pieces.

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