Convert obmc-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.

Tested: Built a Witherspoon image and validated image
Change-Id: I8ab03115bbfc2ecc77cff5c9eb8628903ae88051
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 27bf881..c00930a 100644
--- a/meta-witherspoon/conf/distro/openbmc-witherspoon.conf
+++ b/meta-witherspoon/conf/distro/openbmc-witherspoon.conf
@@ -1,2 +1,3 @@
 require conf/distro/include/phosphor-base.inc
+require conf/distro/include/phosphor-ubi.inc
 require conf/distro/include/openpower-mrw.inc