Move host-ipmi-bt from palmetto to common include

This is the default host-ipmi-hw for all openpower systems.
diff --git a/meta-openbmc-machines/meta-openpower/conf/machine/include/openpower.inc b/meta-openbmc-machines/meta-openpower/conf/machine/include/openpower.inc
index 89ad0ec..10fa40c 100644
--- a/meta-openbmc-machines/meta-openpower/conf/machine/include/openpower.inc
+++ b/meta-openbmc-machines/meta-openpower/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"
diff --git a/meta-openbmc-machines/meta-openpower/meta-ibm/meta-palmetto/conf/machine/palmetto.conf b/meta-openbmc-machines/meta-openpower/meta-ibm/meta-palmetto/conf/machine/palmetto.conf
index f1e4250..f6256a7 100644
--- a/meta-openbmc-machines/meta-openpower/meta-ibm/meta-palmetto/conf/machine/palmetto.conf
+++ b/meta-openbmc-machines/meta-openpower/meta-ibm/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"