tools/buildroot: Add support for generating Linux target system images

Integrate buildroot into devlib in order to ease building kernel and
root filesystem images via 'generate-kernel-initrd.sh' helper script.

As its name suggests, the script builds kernel image which also includes
an initial RAM disk per default config files located under
configs/<arch>/.

Provide config files for buildroot and Linux kernel as well as a
post-build.sh script which tweaks (e.g., allowing root login on SSH)
target's root filesystem.

doc/tools.rst talks about details of kernel and rootfs configuration.

Signed-off-by: Metin Kaya <metin.kaya@arm.com>

doc/index.rst

@@ -25,6 +25,7 @@ Contents:
    derived_measurements
    platform
    connection
+   tools
 
 Indices and tables
 ==================

doc/tools.rst

@@ -41,3 +41,45 @@ Android emulator:
    # After ~30 seconds, the emulated device will be ready:
    adb -s emulator-5554 shell "lsmod"
 
+
+Building buildroot
+------------------
+
+``buildroot/generate-kernel-initrd.sh`` helper script downloads and builds
+``buildroot`` per config files located under ``tools/buildroot/configs``
+for the specified architecture.
+
+The script roughly checks out ``2023.11.1`` tag of ``buildroot``, copies config
+files for buildroot (e.g., ``configs/aarch64/arm-power_aarch64_defconfig``) and
+kernel (e.g., ``configs/aarch64/linux.config``) to necessary places under
+buildroot directory, and runs ``make arm-power_aarch64_defconfig && make``
+commands.
+
+As its name suggests, ``generate-kernel-initrd.sh`` builds kernel image with an
+initial RAM disk per default config files.
+
+There is also ``post-build.sh`` script in order to make following tunings on
+root filesystem generated by ``buildroot``:
+- allow root login on SSH.
+- increase number of concurrent SSH connections/channels to let devlib
+  consumers hammering the target system.
+
+In order to keep rootfs minimal, only OpenSSH and util-linux packages
+are enabled in the default configuration files.
+
+DHCP client and SSH server services are enabled on target system startup.
+
+SCHED_MC, SCHED_SMT and UCLAMP_TASK scheduler features are enabled for aarch64
+kernel.
+
+If you need to make changes on ``buildroot``, rootfs or kernel of target
+system, you may want to run commands similar to these:
+
+.. code:: shell
+
+   $ cd tools/buildroot/buildroot-v2023.11.1-aarch64
+   $ make menuconfig    # or 'make linux-menuconfig' if you want to configure kernel
+   $ make
+
+See https://buildroot.org/downloads/manual/manual.html for details.
+