[PATCHv2,4/4] docs/manual: add explanations about BR2_EXTERNAL

diff --git a/docs/manual/customize-outside-br.txt b/docs/manual/customize-outside-br.txt
new file mode 100644
index 0000000..74899af
--- /dev/null
+++ b/docs/manual/customize-outside-br.txt
@@ -0,0 +1,112 @@ 
+// -*- mode:doc -*- ;
+
+[[outside-br-custom]]
+Keeping customization outside Buildroot
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+While the Buildroot community recommends and encourages upstreaming to
+the official Buildroot version the packages and boards support that
+are written by developers, it is sometimes not possible or desirable,
+due to these packages or boards being highly specific or proprietary.
+
+In this case, Buildroot users are offered two choices:
+
+ * They can add their packages, board support and configuration files
+   directly within the Buildroot tree, and maintain them by using
+   branches in a version control system.
+
+ * They can use the +BR2_EXTERNAL+ mechanism, which allows to keep
+   package recipes, board support and configuration files outside of
+   the Buildroot tree, while still having them nicely integrated in
+   the build logic. The following paragraphs give details on how to
+   use +BR2_EXTERNAL+.
+
++BR2_EXTERNAL+ is an environment variable that one can use to point to
+a directory that contains Buildroot customizations. This environment
+variable must be passed everytime a Buildroot +make+ command is
+invoked, be it at configuration time (+make menuconfig+, +make
+xconfig+, etc.), at build time (+make+) or any other Buildroot command
+(+make source+, +make legal-info+, etc.). As an exception, when
+building out-of-tree with +O=+, the +BR2_EXTERNAL+ value is encoded
+into the Makefile wrapper generated by Buildroot, so that passing
++BR2_EXTERNAL+ is no longer needed.
+
+Examples:
+
+-----
+ buildroot/ $ make BR2_EXTERNAL=../company menuconfig
+ buildroot/ $ make BR2_EXTERNAL=../company
+-----
+
+This +BR2_EXTERNAL+ then allows three different things:
+
+ * One can store all the board-specific configuration files here, such
+   as the kernel configuration, the root filesystem overlay, or any
+   other configuration file for which Buildroot allows to set its
+   location. The +BR2_EXTERNAL+ value is available within the
+   Buildroot configuration using +$(BR2_EXTERNAL)+. As an example, one
+   could set the +BR2_ROOTFS_OVERLAY+ Buildroot option to
+   +$(BR2_EXTERNAL)/board/<boardname>/overlay/+ (to specify a root
+   filesystem overlay), or the +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+
+   Buildroot option to +$(BR2_EXTERNAL)/board/<boardname>/kernel.config+
+   (to specify the location of the kernel configuration file).  + To
+   achieve this, it is recommended but not mandatory, to store those
+   details in directories called +board/<boardname>/+ under
+   +BR2_EXTERNAL+.
+
+ * One can store package recipes (i.e +Config.in+ and
+   +<packagename>.mk+), or even custom configuration options and make
+   logic. Buildroot automatically includes +BR2_EXTERNAL/Config.in+ to
+   make it appear in the top-level configuration menu, and includes
+   +BR2_EXTERNAL/external.mk+ with the rest of the makefile logic.
++
+The main usage of this is to store package recipes. The recommended
+   way to do this is to write a +BR2_EXTERNAL/Config.in+ that looks
+   like:
++
+------
+menu "<somecompany> packages"
+
+source "$BR2_EXTERNAL/package/package1/Config.in"
+source "$BR2_EXTERNAL/package/package2/Config.in"
+
+endmenu
+------
++
+Then, have a +BR2_EXTERNAL/external.mk' file that looks like:
++
+------
+include $(BR2_EXTERNAL)/package/*/*.mk
+------
++
+And then in +BR2_EXTERNAL/package/package1+ and
+   +BR2_EXTERNAL/package/package2+ create normal Buildroot package
+   recipes, as explained in xref:adding-packages[].
+
+ * One can store Buildroot defconfigs in the +configs+ subdirectory of
+   +BR2_EXTERNAL+. Buildroot will automatically show them in the
+   output of +make help+ and allow them to be loaded with the normal
+   +make <name>_defconfig+ command.
+
+In the end, a typical +BR2_EXTERNAL+ directory organization would
+generally be:
+
+-----
+├── Config.in
+├── external.mk
+├── board/
+│   └── <boardname>/
+│       └── overlay/
+│           └── etc/
+│               └── <some file>
+├── configs/
+│   └── <boardname>_defconfig
+└── package/
+    ├── Config.in
+    └── package1/
+        ├── Config.in
+        └── package1.mk
+    └── package2/
+        ├── Config.in
+        └── package2.mk
+------
diff --git a/docs/manual/customize.txt b/docs/manual/customize.txt
index 0456ef1..7e46fd8 100644
--- a/docs/manual/customize.txt
+++ b/docs/manual/customize.txt
@@ -17,3 +17,5 @@  include::customize-toolchain.txt[]
 include::customize-store.txt[]
 
 include::customize-packages.txt[]
+
+include::customize-outside-br.txt[]