buildroot

manual/user guide/customization: add section on device/permission tables

This patch adds basic documentation on device and permission tables, a topic
which was currently not yet covered in the manual.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>