[Buildroot] [PATCH 3/3] docs/manual: add explanations about BR2_EXTERNAL

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

This commit updates the manual to add details on how to use the
BR2_EXTERNAL feature.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---
 docs/manual/customize-outside-br.txt | 110 +++++++++++++++++++++++++++++++++++
 docs/manual/customize.txt            |   2 +
 2 files changed, 112 insertions(+)
 create mode 100644 docs/manual/customize-outside-br.txt

diff --git a/docs/manual/customize-outside-br.txt b/docs/manual/customize-outside-br.txt
new file mode 100644
index 0000000..81a8911
--- /dev/null
+++ b/docs/manual/customize-outside-br.txt
@@ -0,0 +1,110 @@
+// -*- 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.).
+
+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/someboard/overlay/+ (to specify a root
+   filesystem overlay), or the +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+
+   Buildroot option to +$(BR2_EXTERNAL)/board/someboard/kernel.config+
+   (to specify the location of the kernel configuration file).
++
+To achieve this, it is recommended but not mandatory, to store those
+   details in directories called +board/<boardname>/+ under
+   +BR2_EXTERNAL+.
+
+ * One can store package recipes (i.e +Config.in+ and
+   +<packagename>.mk+), or even custom configuration options and make
+   logic. Buildroot automatically includes +BR2_EXTERNAL/Config.in+ to
+   make it appear in the top-level configuration menu, and includes
+   +BR2_EXTERNAL/external.mk+ with the rest of the makefile logic.
++
+The main usage of this would be to store package recipes. The
+   recommended way to do this is to write a +BR2_EXTERNAL/Config.in+
+   that looks like:
++
+------
+menu "<somecompany> packages"
+
+source "$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/
+?   ??? <someboard>/
+?       ??? overlay/
+?           ??? etc/
+?               ??? <some file>
+??? configs/
+?   ??? <some board>_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[]
-- 
1.8.1.2