arm-autonomy/xenguest-mkimage: add extra ramdisk support

Some guest images requires a ramdisk, that is built with a custom recipe.
This patch adds an option '--xen-ramdisk',
for xenguest-mkimage script to cover that.
Extra ramdisk support is also added to xenguest-image-extra bbclass,
via 'XENGUEST_EXTRA_RAMDISK' variable.

Issue-Id: SCM-1276
Change-Id: I8ec74a37c0c96ca83a3489911186f3b3262f80fd
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>