docs: Add docs for qemuarm64 build

In order to have qemu arm support, add information on how to build and
run a base image for qemuarm64

Related-to docs#126



Signed-off-by: Luca Seritan <luca.seritan@huawei.com>

docs/hardware-support/virtual-boards/qemuarm64.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: ../../definitions.rst
+
+Qemu arm64
+##########
+
+.. contents:: 
+   :depth: 4
+
+Overview
+********
+
+|main_project_name| supports running the software stack into an virtual environment using Qemu.
+
+Building an Oniro image
+=======================
+
+To clone the source code, perform the procedure in: :ref:`Setting up a repo workspace <RepoWorkspace>`.
+
+Building a Linux image
+======================
+
+Build Steps
+-----------
+
+1. Source the environment with proper template settings, flavour being *linux*
+   and target machine being *qemuarm64*. Pay attention to how relative paths are
+   constructed. The value of *TEMPLATECONF* is relative to the location of the
+   build directory *./build-oniro-linux*, that is going to be created after
+   this step:
+
+.. code-block:: console
+
+   $ TEMPLATECONF=../oniro/flavours/linux . ./oe-core/oe-init-build-env build-oniro-linux
+
+2. You will find yourself in the newly created build directory. Call *bitbake*
+   to build the image. For example, if you are using *oniro-image-base*
+   run the following command:
+
+.. code-block:: console
+
+   $ MACHINE=qemuarm64 bitbake oniro-image-base
+
+Once the image is done, you can run the Qemu using the provided script wrapper:
+
+.. code-block:: console
+
+   $ MACHINE=qemuarm64 runqemu oniro-image-base ext4 slirp