automation: Change build script to use arch defconfig

Change automation build script to call the make defconfig target before
setting CONFIG_DEBUG and extra options. This fixes issues on Power where
the build fails without using the ppc64_defconfig.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

automation/scripts/build

@@ -22,7 +22,10 @@ if [[ "${RANDCONFIG}" == "y" ]]; then
     # RANDCONFIG implies HYPERVISOR_ONLY
     HYPERVISOR_ONLY="y"
 else
-    echo "CONFIG_DEBUG=${debug}" > xen/.config
+    # Start off with arch's defconfig
+    make -C xen defconfig
+
+    echo "CONFIG_DEBUG=${debug}" >> xen/.config
 
     if [[ -n "${EXTRA_XEN_CONFIG}" ]]; then
         echo "${EXTRA_XEN_CONFIG}" >> xen/.config