arm-autonomy/documentation: Remove references to meta-kernel

Since meta-arm-bsp doesn't depend on meta-kernel anymore
it has been removed from the appropriate documentation.

Issue-Id: SCM-2005
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: If3548b84c4cffc5cc105a1fc07a656dc814699f3
Signed-off-by: Jon Mason <jon.mason@arm.com>