arm-autonomy: Create arm-autonomy-host feature

Create a new distribution feature to enable functionalities for autonomy
host systems:
- enable xen and ipv4 in distribution features
- enable xen backend drivers in Linux kernel using a kmeta feature

For the feature to work properly, create a class inherited when the
layer is included to enable required parameters

Change-Id: Ia0cdaa2b69bc030515ea16d67c47fec6b1ea0897
Issue-Id: SCM-767
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-autonomy/recipes-kernel/linux/linux-%.bbappend

@@ -0,0 +1,3 @@
+# Add arm-autonomy kernel support
+require linux-arm-autonomy.inc
+