manual/user guide/customization: add section with recommended dir structure

Although the user is free to choose where to put project-specific files, the
presentation 'Using Buildroot for real projects' [1] first described a
certain recommendation.
This patch briefly describes this recommendation in the form of a directory
structure overview, the details will be added in subsequent patches.

Since the section on BR2_EXTERNAL is logically connected to the directory
structure, this section is moved right after it.

[1] http://elinux.org/images/2/2a/Using-buildroot-real-project.pdf

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/customize-directory-structure.txt

@@ -0,0 +1,58 @@
+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+=== Recommended directory structure
+
+When customizing Buildroot for your project, you will be creating one or
+more project-specific files that need to be stored somewhere. While most
+of these files could be placed in _any_ location as their path is to be
+specified in the Buildroot configuration, the Buildroot developers
+recommend a specific directory structure which is described in this
+section.
+
+Orthogonal to this directory structure, you can choose _where_ you place
+this structure itself: either inside the Buildroot tree, or outside of
+it using +BR2_EXTERNAL+. Both options are valid, the choice is up to you.
+
+-----
++-- board/
+|   +-- <company>/
+|       +-- <boardname>/
+|           +-- linux.config
+|           +-- busybox.config
+|           +-- <other configuration files>
+|           +-- post_build.sh
+|           +-- post_image.sh
+|           +-- rootfs_overlay/
+|           |   +-- etc/
+|           |   +-- <some file>
+|           +-- patches/
+|               +-- foo/
+|               |   +-- <some patch>
+|               +-- libbar/
+|                   +-- <some other patches>
+|
++-- configs/
+|   +-- <boardname>_defconfig
+|
++-- package/
+|   +-- <company>/
+|       +-- Config.in (if not using BR2_EXTERNAL)
+|       +-- <company>.mk (if not using BR2_EXTERNAL)
+|       +-- <boardname>/
+|           +-- package1/
+|           |    +-- Config.in
+|           |    +-- package1.mk
+|           +-- package2/
+|               +-- Config.in
+|               +-- package2.mk
+|
++-- Config.in (if using BR2_EXTERNAL)
++-- external.mk (if using BR2_EXTERNAL)
+------
+
+Details on the files shown above are given further in this chapter.
+
+Note: if you choose to place this structure outside of the Buildroot
+tree using +BR2_EXTERNAL+, the <company> and possibly <boardname>
+components may be superfluous and can be left out.

docs/manual/customize.txt

@@ -26,10 +26,12 @@ image in a reproducible way, even after running 'make clean'. By
 following the recommended strategy, you can even use the same Buildroot
 tree to build multiple distinct projects!
 
+include::customize-directory-structure.txt[]
+
+include::customize-outside-br.txt[]
+
 include::customize-rootfs.txt[]
 
 include::customize-store.txt[]
 
 include::customize-packages.txt[]
-
-include::customize-outside-br.txt[]