arm: use module provided command line for domain 0 command line

arm: use module provided command line for domain 0 command line

Fallback to xen,dom0-bootargs if this isn't present.

Ideally this would use module1-args iff the kernel came from the
modules and the existing xen,dom0-bootargs if the kernel came from
flash, but this approach is simpler and has the same effect in
practice.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>