ewaol-bsp/xen/ava: Fix Dom0 earlycon kernel command line parameter

Update the xen.cfg.in file used when building with 'xen-pci-passthrough' in
MACHINE_FEATURES to have earlycon=xenboot for Dom0 kernel command line
otherwise early messages are not printed into the console.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I902e6585beeb184669e5109c386365ad280a7014