system: add boot options (from old wiki)

The following options have not been migrated because we
did not find its valid and correct use:

no-vyos-configure
    Do not load the config, leaves you with plain Linux system.

no-vyos-rl-system
    Disables getting the system into clean state before loading the config.

docs/appendix/troubleshooting.rst

@@ -301,6 +301,8 @@ to clear counters on firewall rulesets or single rules
 Basic System Information
 ------------------------
 
+.. _boot-steps:
+
 Boot steps
 ^^^^^^^^^^
 

docs/system/boot-options.rst

@@ -0,0 +1,56 @@
+.. _boot-options:
+
+
+############
+Boot Options
+############
+
+.. warning:: This function may be highly disruptive.
+   It may cause major service interruption, so make sure you really
+   need it and verify your input carefully.
+
+
+
+VyOS has several kernel command line options to modify the normal boot
+process. 
+To add an option, select the desired image in GRUB menu at load
+time, press **e**, edit the first line, and press **Ctrl-x** to boot when
+ready.
+
+.. image:: /_static/images/boot-options.png
+   :width: 80%
+   :align: center
+
+
+Specify custom config file
+==========================
+
+Tells the system to use specified file instead of ``/config/config.boot``.
+If specified file does not exist or is not readable, fall back to
+default config. No additional verification is performed, so make sure
+you specify a valid config file.
+
+.. code-block:: none
+
+   vyos-config=/path/to/file
+
+To load the *factory default* config, use:
+
+.. code-block:: none
+
+   vyos-config=/opt/vyatta/etc/config.boot.default
+
+
+Disable specific boot process steps
+===================================
+
+These options disable some boot steps. Make sure you understand the
+:ref:`boot process <boot-steps>` well before using them!
+
+.. glossary::
+
+    no-vyos-migrate
+      Do not perform config migration.
+
+    no-vyos-firewall
+      Do not initialize default firewall chains, renders any firewall configuration unusable.

docs/system/index.rst

@@ -22,3 +22,4 @@ should be ready for further configuration which is described in this chapter.
    syslog
    task-scheduler
    time-zone
+   boot-options