Doc: Add tutorial about using zephyr as uos

This patch add tutorial about using zephyr as guest.

Tracked-On: #2713
Signed-off-by: Kaige Fu <kaige.fu@intel.com>

---
v3 -> v4:
  - Addressed two minor comments from David

v2 -> v3:
  - Detail section `Build Zephyr`
  - Add missing sudo
  - Automatically boot zephyr
  - Other grammar, phrasing fix

v1 -> v2:
  - NUC7i5DNH -> NUC7i5DNHE.
  - Add note about doc to build acrn supported zephyr image.
  - Refine section `Boot Zephyr as User OS`.
  - Some minor fix.

doc/tutorials/using_zephyr_as_uos.rst

@@ -0,0 +1,113 @@
+.. _using_zephyr_as_uos:
+
+Using Zephyr as User OS
+#######################
+
+This tutorial describes how to run Zephyr as the User OS on the ACRN hypervisor. We are using
+Kaby Lake-based NUC (model NUC7i5DNHE) in this tutorial.
+Other :ref:`ACRN supported platforms <hardware>` should work as well.
+
+Introduction to Zephyr
+**********************
+
+The Zephyr RTOS is a scalable real-time operating-system supporting multiple hardware architectures,
+optimized for resource constrained devices, and built with safety and security in mind.
+
+Steps for Using Zephyr as User OS
+*********************************
+
+#. Build Zephyr
+
+   Follow the `Zephyr Getting Started Guide <https://docs.zephyrproject.org/latest/getting_started/>`_ to
+   setup the Zephyr development environment.
+
+   The build process for ACRN UOS target is similar to other boards. We will build the `Hello World
+   <https://docs.zephyrproject.org/latest/samples/hello_world/README.html>`_ sample for ACRN:
+
+   .. code-block:: none
+
+      $ cd samples/hello_world
+      $ mkdir build && cd build
+      $ cmake -DBOARD=acrn ..
+      $ make
+
+   This will build the application ELF binary in ``samples/hello_world/build/zephyr/zephyr.elf``.
+
+#. Build grub2 boot loader image
+
+   We can build grub2 bootloader for Zephyr using ``boards/x86/common/scripts/build_grub.sh``
+   which locate in `Zephyr Sourcecode <https://github.com/zephyrproject-rtos/zephyr>`_.
+
+   .. code-block:: none
+
+      $ ./boards/x86/common/scripts/build_grub.sh x86_64
+
+   The EFI executable binary will be found at ``boards/x86/common/scripts/grub/bin/grub_x86_64.efi``.
+
+#. Preparing the boot device
+
+   .. code-block:: none
+
+      $ dd if=/dev/zero of=zephyr.img bs=1M count=35
+      $ mkfs.vfat -F 32 zephyr.img
+      $ sudo mount `sudo losetup -f -P --show zephyr.img` /mnt
+
+   Create the following directories.
+
+   .. code-block:: none
+
+      $ sudo mkdir -p /mnt/efi/boot
+      $ sudo mkdir -p /mnt/kernel
+
+   Copy ``zephyr.elf`` and ``grub_x86_64.efi``
+
+   .. code-block:: none
+
+      $ sudo cp boards/x86/common/scripts/grub/bin/grub_x86_64.efi /mnt/efi/boot/bootx64.efi
+      $ sudo cp samples/hello_world/build/zephyr/zephyr.elf /mnt/kernel
+
+   Create ``/mnt/efi/boot/grub.cfg`` containing the following:
+
+   .. code-block:: console
+
+      set default=0
+      set timeout=10
+
+      menuentry "Zephyr Kernel" {
+          multiboot /kernel/zephyr.elf
+      }
+
+   Unmount the loopback device:
+
+   .. code-block:: none
+
+      $ sudo umount /mnt
+
+   You now have a virtual disk image with a bootable Zephyr in ``zephyr.img``. If the Zephyr build system is not
+   the ACRN SOS, then you will need to transfer this image to the ACRN SOS (via, e.g, a USB stick or network )
+
+#. Follow :ref:`getting-started-apl-nuc` to boot "The ACRN Service OS"
+
+#. Boot Zephyr as User OS
+
+   On the ACRN SOS, prepare a directory and populate it with Zephyr files.
+
+   .. code-block:: none
+
+      $ mkdir zephyr && cd zephyr
+      $ cp /usr/share/acrn/samples/nuc/launch_zephyr.sh .
+      $ cp /usr/share/acrn/bios/OVMF.fd .
+
+   You will also need to copy the ``zephyr.img`` created in the above section into directory ``zephyr``.
+
+   Run the ``launch_zephyr.sh`` script to launch Zephyr as UOS.
+
+   .. code-block:: none
+
+      $ sudo ./launch_zephyr.sh
+
+   Then Zephyr will boot up automatically. You will see a console message from the hello_world sample application:
+
+   .. code-block:: console
+
+      Hello World! acrn