arm-autonomy: Use busybox instead of GNU tar

Switch out the tar used by xenguest-manager and xenguest-mkimage.
xenguest-mkimage requires the --exclude flag so enable the Busybox
CONFIG_FEATURE_TAR_LONG_OPTIONS option.

Issue-Id: SCM-2034
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Iceed017ef8ebd058e6c06156989aaa2a4bff9103
Signed-off-by: Jon Mason <jon.mason@arm.com>