mbox: Use MACHINE_FEATURES for virtual-pnor

Define MACHINE_FEATURE openpower-ubi-fs to enable the
virtual-pnor functionality and enable on Witherspoon.

Change-Id: I57a6a39cf36501cf6c638df13b4e4b08a70fec26
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/common/recipes-phosphor/mboxd/mboxd.bb b/common/recipes-phosphor/mboxd/mboxd.bb
index 8915dd7..b6118e4 100644
--- a/common/recipes-phosphor/mboxd/mboxd.bb
+++ b/common/recipes-phosphor/mboxd/mboxd.bb
@@ -46,6 +46,6 @@
 SYSTEMD_SERVICE_${PN} += "mboxd-reload@.service"
 SYSTEMD_LINK_${PN} += "${@compose_list(d, 'FMT', 'OBMC_CHASSIS_INSTANCES')}"
 
-# Disable virtual-pnor by default.
-PACKAGECONFIG ??= ""
+# Enable virtual-pnor by MACHINE_FEATURE openpower-ubi-fs.
+PACKAGECONFIG_append = "${@mf_enabled(d, 'openpower-ubi-fs', 'virtual-pnor')}"
 PACKAGECONFIG[virtual-pnor] = "--enable-virtual-pnor,--disable-virtual-pnor"