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>

docs/manual/customize-device-permission-tables.txt

@@ -0,0 +1,30 @@
+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+[[customize-device-permission]]
+==== Setting file permissions and ownership and adding custom devices nodes
+
+Sometimes it is needed to set specific permissions or ownership on files
+or device nodes. For example, certain files may need to be owned by
+root.  Since the post-build scripts are not run as root, you cannot do
+such changes from there unless you use an explicit fakeroot from the
+post-build script.
+
+Instead, Buildroot provides support for so-called _permission tables_.
+To use this feature, set config option +BR2_ROOTFS_DEVICE_TABLE+ to a
+space-separated list of permission tables, regular text files following
+the xref:makedev-syntax[makedev syntax].
+
+If you are using a static device table (i.e. not using +devtmpfs+,
++mdev+, or +(e)udev+) then you can add device nodes using the same
+syntax, in so-called _device tables_. To use this feature, set config
+option +BR2_ROOTFS_STATIC_DEVICE_TABLE+ to a space-separated list of
+device tables.
+
+As shown in xref:customize-dir-structure[], the recommended location for
+such files is +board/<company>/<boardname>/+.
+
+It should be noted that if the specific permissions or device nodes are
+related to a specific application, you should set variables
++FOO_PERMISSIONS+ and +FOO_DEVICES+ in the package's +.mk+ file instead
+(see xref:generic-package-reference[]).

docs/manual/customize-rootfs.txt

@@ -97,3 +97,5 @@ To enable this feature, enable config option
 This method is not recommended because it duplicates the entire
   skeleton, which prevents taking advantage of the fixes or improvements
   brought to the default skeleton in later Buildroot releases.
+
+include::customize-device-permission-tables.txt[]

docs/manual/customize.txt

@@ -16,6 +16,10 @@ Typical actions you may need to perform for a given project are:
      +BR2_ROOTFS_POST_BUILD_SCRIPT+)
   ** running arbitrary commands prior to generating the filesystem image
      (using +BR2_ROOTFS_POST_BUILD_SCRIPT+)
+  ** setting file permissions and ownership (using
+     +BR2_ROOTFS_DEVICE_TABLE+)
+  ** adding custom devices nodes (using
+     +BR2_ROOTFS_STATIC_DEVICE_TABLE+)
 * running arbitrary commands after generating the filesystem image
   (using +BR2_ROOTFS_POST_IMAGE_SCRIPT+)
 * adding project-specific patches to some packages (using