Move host-ipmi-bt from palmetto to common include

This is the default host-ipmi-hw for all openpower systems.
diff --git a/conf/machine/include/openpower.inc b/conf/machine/include/openpower.inc
index 89ad0ec..10fa40c 100644
--- a/conf/machine/include/openpower.inc
+++ b/conf/machine/include/openpower.inc
@@ -17,3 +17,5 @@
 VIRTUAL-RUNTIME_obmc-phosphor-flash-ctl = " \
        obmc-phosphor-example-pydbus \
        "
+
+PREFERRED_PROVIDER_virtual/obmc-phosphor-host-ipmi-hw = "host-ipmi-bt"