Convert openpower-ubi-fs to a distro feature

As with MRW the use of the UBI image scheme is distro policy.
Convert the existing machine feature to a distro feature.

Enable the new distro feature on the systems that use
it(Witherspoon).

Add a distro override and fix-up MF checks to use this override
instead for improved readability.

Change-Id: I57f3bc244523eff5bc043caff8365e334d98cb30
Tested: Built a witherspoon image and validate manifest
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/common/recipes-phosphor/mboxd/mboxd.bb b/common/recipes-phosphor/mboxd/mboxd.bb
index d60c23a..f90ab86 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')}"
 
-# Enable virtual-pnor by MACHINE_FEATURE openpower-ubi-fs.
-PACKAGECONFIG_append = "${@mf_enabled(d, 'openpower-ubi-fs', 'virtual-pnor')}"
+# Enable virtual-pnor by DISTRO_FEATURE openpower-ubi-fs.
+PACKAGECONFIG_append_df-openpower-ubi-fs = "virtual-pnor"
 PACKAGECONFIG[virtual-pnor] = "--enable-virtual-pnor,--disable-virtual-pnor"