Switch the default build to make the -xen kernel, not the -xen0 and -xenU

kernels.  Distros will be using -xen exclusively, so this change means that
developers and users of unstable will be (by default) using the same
configuration as the distros.

This can be overridden by setting KERNELS="linux-2.6-xen0 linux-2.6-xenU" on the
command line.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

Makefile

@@ -2,7 +2,7 @@
 # Grand Unified Makefile for Xen.
 #
 
-KERNELS ?= linux-2.6-xen0 linux-2.6-xenU
+KERNELS ?= linux-2.6-xen
 # You may use wildcards in the above e.g. KERNELS=*2.6*
 
 XKERNELS := $(foreach kernel, $(KERNELS), $(patsubst buildconfigs/mk.%,%,$(wildcard buildconfigs/mk.$(kernel))) )