Handle kernel boot options

The 'system.kernel.elevator' and 'system.kernel.isolcpu' options
have been kept for backward compatibility and should be used in new
fashion way with system.kernel.boot_options parameter.

Change-Id: I51f7167b8b8946500df2065ee6b02bcf21809bc9
diff --git a/README.rst b/README.rst
index 8628f80..6f67de2 100644
--- a/README.rst
+++ b/README.rst
@@ -407,6 +407,18 @@
             net.ipv4.tcp_keepalive_time: 30
             net.ipv4.tcp_keepalive_probes: 8
 
+Configure kernel boot options:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        kernel:
+          boot_options:
+            - elevator=deadline
+            - spectre_v2=off
+            - nopti
+
 
 CPU
 ~~~