Capture kernel boot messages in journal. (#1114)

Closes #1113

Signed-off-by: Bun K Tan <bun.k.tan@intel.com>

source/guides/kernel/kernel-boot-msg.rst

@@ -0,0 +1,69 @@
+.. _kernel-boot-msg:
+
+Capture Kernel Boot Messages in the Journal
+###########################################
+
+By default |CL| does not capture kernel boot messages in the journal logs, 
+where they're reported as "Missed" messages.  This design decision was made 
+to provide a faster boot performance.  On the other hand, if you wish to 
+see the messages, follow this guide.
+
+Here's an example a journal log with "Missed" messages:
+
+.. code-block:: console
+   :linenos:
+   :emphasize-lines: 4
+
+   -- Reboot --
+   Apr 10 19:55:43 kernel systemd-journald[300]: Journal started
+   Apr 10 19:55:43 kernel systemd-journald[300]: Runtime Journal (/run/log/journal/d01862ca79d1064ea379cd715cfdd53a) is 5.8M, max 47.0M, 41.1M free.
+   Apr 10 19:55:43 kernel systemd-journald[300]: Missed 2233 kernel messages
+   Apr 10 19:55:43 kernel systemd[1]: Started Journal Service.
+
+.. contents::
+   :local:
+   :depth: 1
+ 
+Prerequisites
+*************
+
+* `systemd-journald` version 245 and higher 
+
+Enable journaling of kernel boot messages
+*****************************************
+
+#. Open a terminal window.
+
+#. Create a base journald configuration file.
+
+   .. code-block:: bash
+
+      sudo mkdir -p /etc/systemd/journald.conf.d
+      sudo cp /usr/lib/systemd/journald.conf.d/clear.conf /etc/systemd/journald.conf.d/
+
+#. Append :command:`BootKMsg=true` to it.
+   
+   .. code-block:: bash
+
+      echo "BootKMsg=true" | sudo tee -a /etc/systemd/journald.conf.d/clear.conf
+
+#. Reboot.  
+
+.. tip::
+
+   If you need to increase the kernel buffer length (for example, 1M), do this: 
+
+   .. code-block:: bash
+   
+      sudo mkdir -p /etc/kernel/cmdline.d/
+      echo "log_buf_len=1M" | sudo tee /etc/kernel/cmdline.d/log_buf_len.conf
+      sudo clr-boot-manager update
+
+Alternative
+***********
+
+An alternative is to use :command:`dmesg`.  
+
+.. code-block:: bash
+
+   sudo dmesg