doc: add steps for modifying configs and generating defconfigs

This patch adds two sections which describe how configurations can be modified
and how minimized configurations can be generated.

Also switch prerequisite python version from 2 to 3 since menuconfig.py is
python3 only.

v1 -> v2:

    * Add the link to EPEL.
    * Rename minimalconfig to savedefconfig in the guide.
    * Split the step using a defconfig from those generating a defconfig.

Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>
Acked-by: Geoffroy VanCutsem <geoffroy.vancutsem@intel.com>

doc/getting_started/index.rst

@@ -375,14 +375,18 @@ Install build tools and dependencies
 ARCN development is supported on popular Linux distributions,
 each with their own way to install development tools:
 
+  .. note::
+     ACRN uses ``menuconfig``, a python3 text-based user interface (TUI) for
+     configuring hypervisor options and using python's ``kconfiglib`` library.
+
 * On a Clear Linux development system, install the ``os-clr-on-clr`` bundle to get
   the necessary tools:
 
   .. code-block:: console
 
      $ sudo swupd bundle-add os-clr-on-clr
-     $ sudo swupd bundle-add python-basic
-     $ sudo pip install kconfiglib
+     $ sudo swupd bundle-add python3-basic
+     $ sudo pip3 install kconfiglib
 
 * On a Ubuntu/Debian development system:
 
@@ -397,9 +401,9 @@ each with their own way to install development tools:
           libevent-dev \
           libxml2-dev \
           libusb-1.0-0-dev \
-          python \
-          python-pip
-     $ sudo pip install kconfiglib
+          python3 \
+          python3-pip
+     $ sudo pip3 install kconfiglib
 
   .. note::
      Ubuntu 14.04 requires ``libsystemd-journal-dev`` instead of ``libsystemd-dev``
@@ -418,9 +422,9 @@ each with their own way to install development tools:
           libxml2-devel \
           libevent-devel \
           libusbx-devel \
-          python \
-          python-pip
-     $ sudo pip install kconfiglib
+          python3 \
+          python3-pip
+     $ sudo pip3 install kconfiglib
 
 
 * On a CentOS development system:
@@ -436,9 +440,15 @@ each with their own way to install development tools:
              libxml2-devel \
              libevent-devel \
              libusbx-devel \
-             python \
-             python-pip
-     $ sudo pip install kconfiglib
+             python34 \
+             python34-pip
+     $ sudo pip3 install kconfiglib
+
+  .. note::
+     You may need to install `EPEL <https://fedoraproject.org/wiki/EPEL>`_ for
+     installing python3 via yum for CentOS 7. For CentOS 6 you need to install
+     pip manually. Please refer to https://pip.pypa.io/en/stable/installing for
+     details.
 
 
 Build the hypervisor, device model and tools
@@ -496,3 +506,76 @@ and are using it as the current working directory.
 
 Follow the same instructions to boot and test the images you created
 from your build.
+
+Generate the hypervisor configurations
+======================================
+
+The ACRN hypervisor leverages Kconfig to manage configurations, powered by
+Kconfiglib. A default configuration is generated based on the platform you have
+selected via the ``PLATFORM=`` command line parameter. You can make further
+changes to that default configuration to adjust to your specific
+requirements.
+
+To generate hypervisor configurations, you need to build the hypervisor
+individually. The following steps generate a default but complete configuration,
+based on the platform selected, assuming that you are under the top-level
+directory of acrn-hypervisor. The configuration file, named ``.config``, can be
+found under the target folder of your build.
+
+   .. code-block:: console
+
+      $ cd hypervisor
+      $ make defconfig PLATFORM=uefi
+
+The PLATFORM specified is used to select a defconfig under
+``arch/x86/configs/``. The other command-line based options (e.g. ``RELEASE``)
+take no effects when generating a defconfig.
+
+Modify the hypervisor configurations
+====================================
+
+To modify the hypervisor configurations, you can either edit ``.config``
+manually, or invoke a TUI-based menuconfig, powered by kconfiglib, by executing
+``make menuconfig``. As an example, the following commands, assuming that you
+are under the top-level directory of acrn-hypervisor, generate a default
+configuration file for UEFI, allow you to modify some configurations and build
+the hypervisor using the updated ``.config``.
+
+   .. code-block:: console
+
+      $ cd hypervisor
+      $ make defconfig PLATFORM=uefi
+      $ make menuconfig              # Modify the configurations per your needs
+      $ make                         # Build the hypervisor with the new .config
+
+   .. note::
+      Menuconfig is python3 only.
+
+Refer to the help on menuconfig for a detailed guide on the interface.
+
+   .. code-block:: console
+
+      $ pydoc3 menuconfig
+
+Create a new default configuration
+==================================
+
+Currently the ACRN hypervisor looks for default configurations under
+``hypervisor/arch/x86/configs/<PLATFORM>.config``, where ``<PLATFORM>`` is the
+specified platform. The following steps allow you to create a defconfig for
+another platform based on a current one.
+
+   .. code-block:: console
+
+      $ cd hypervisor
+      $ make defconfig PLATFORM=uefi
+      $ make menuconfig         # Modify the configurations
+      $ make savedefconfig      # The minimized config reside at build/defconfig
+      $ cp build/defconfig arch/x86/configs/xxx.config
+
+Then you can re-use that configuration by passing the name (``xxx`` in the
+example above) to 'PLATFORM=':
+
+   .. code-block:: console
+
+      $ make defconfig PLATFORM=xxx