armv8r64-extras/conf: BBMASK sysvinit bbappend in meta-virtualization

meta-virtualization assumes that the Xen console is on hvc0, so adds a
wrapper script which redirects the console to this device. This is not
the correct behavior for Xen dom0less as the UART is passed through
directly to the guest. Therefore, add a BBMASK in layer.conf which
disables this bbappend for the fvp-baser-aemv8r64.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Idc695805e6e3392f1b44bd6823ba39e0bb063a3c