Move host-ipmi-bt from palmetto to common include

This is the default host-ipmi-hw for all openpower systems.
diff --git a/meta-palmetto/conf/machine/palmetto.conf b/meta-palmetto/conf/machine/palmetto.conf
index f1e4250..f6256a7 100644
--- a/meta-palmetto/conf/machine/palmetto.conf
+++ b/meta-palmetto/conf/machine/palmetto.conf
@@ -10,5 +10,3 @@
 UBOOT_LOADADDRESS = "0x40008000"
 
 FLASH_SIZE = "32768"
-
-PREFERRED_PROVIDER_virtual/obmc-phosphor-host-ipmi-hw = "host-ipmi-bt"