user/exe: Add Device Tree section

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+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2019 Vijay Kumar Banerjee <vijaykumar9597@gmail.com>
+
+Device Tree
+===========
+.. index:: Device Tree
+
+A Device Tree is a data structure that is used to describe properties of
+non-discoverable hardware instead of hardcoding them in the kernel. The device
+tree data is generally stored in a `.dts` or a Device Tree Source (DTS) file.
+This file is then compiled into a binary format called Device Tree Blob (DTB)
+with `.dtb` extension. RTEMS preferably uses a DTB built from the FreeBSD source
+tree matching the freebsd-org HEAD commit hash in libBSD.
+
+Building the DTB
+----------------
+
+A single DTB file can be built using the `dtc` tool in libfdt using the
+following command:
+
+.. code-block:: none
+
+    dtc -@ -I dts -O dtb -o my-devicetree.dtb my-devicetree.dts
+
+For building the DTB from the FreeBSD source, the `make_dtb.sh` script
+from `freebsd/sys/tools/fdt` must be used as most of the DTS files in FreeBSD
+have included `.dtsi` files from their source tree. An example is given below as
+a reference for how to build the device tree from the FreeBSD source.
+
+`NOTE: The following example uses FreeBSD master branch from github mirror as
+an example. It is advised to always use the source from the commit matching the
+freebsd-org HEAD in libBSD.`
+
+.. code-block:: shell
+   :linenos:
+
+     #We're using the script from freebsd/sys/tools/make_dtb.sh
+     #Target device: Beaglebone Black.
+     #Architecture: Arm.
+     #DTS source name: am335x-boneblack.dts
+
+     #The make_dtb.sh script uses environment variable MACHINE
+     export MACHINE='arm'
+
+     SCRIPT_DIR=$HOME/freebsd/sys/tools/fdt
+
+     #The arguments to the script are
+     # $1 -> Build Tree (This is the path to freebsd/sys/ directory)
+     # $2 -> DTS source file
+     # $3 -> output path of the DTB file
+
+     ${SCRIPT_DIR}/make_dtb.sh ${SCRIPT_DIR}/../../ \
+     ${SCRIPT_DIR}/../../gnu/dts/arm/am335x-boneblack.dts \
+     $(pwd)
+
+Using Device Tree Overlay
+-------------------------
+
+Device tree overlay is used either to add properties or devices to the existing
+device tree. Adding any property to DTS using an overlay will override the
+current values in the DTB. The Overlays enable us to modify the device tree
+using a small maintainable plugin without having to edit the whole Base Tree.
+
+There are two ways of applying an overlay on top of the built DTB.
+
+#. Use fdtoverlay from libfdt
+
+#. Add the overlay in the root partition of the SD card and apply it using U-Boot
+
+The fdtoverlay command can be used as follows:
+
+.. code-block:: none
+
+    fdtoverlay -i my-base-tree.dtb -o output-tree.dtb my-overlay.dtbo
+
+To apply it from U-Boot during system initialization we have to add the device
+tree overlay file in the root directory of the SD card and use U-Boot commands
+to apply the overlay.
+
+Below is given the series of U-Boot commands that can be used to apply the
+overlay, given that the overlay blob (.dtbo) file is already in the card.
+
+.. code-block:: shell
+
+    fatload mmc 0:1 0x80800000 rtems-app.img
+    fatload mmc 0:1 0x88000000 my-base-tree.dtb
+    fdt addr 0x88000000
+    fatload mmc 0:1 0x88100000 my-overlay.dtbo
+    fdt resize 0x1000
+    fdt apply 0x88100000
+    bootm 0x80800000-0x88000000