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/meta-witherspoon/conf/distro/openbmc-witherspoon.conf b/meta-witherspoon/conf/distro/openbmc-witherspoon.conf
index c00930a..543e7f9 100644
--- a/meta-witherspoon/conf/distro/openbmc-witherspoon.conf
+++ b/meta-witherspoon/conf/distro/openbmc-witherspoon.conf
@@ -1,3 +1,4 @@
 require conf/distro/include/phosphor-base.inc
 require conf/distro/include/phosphor-ubi.inc
 require conf/distro/include/openpower-mrw.inc
+require conf/distro/include/openpower-ubi.inc