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-phosphor/conf/distro/include/phosphor-ubi.inc b/meta-phosphor/conf/distro/include/phosphor-ubi.inc
new file mode 100644
index 0000000..6160446
--- /dev/null
+++ b/meta-phosphor/conf/distro/include/phosphor-ubi.inc
@@ -0,0 +1,2 @@
+DISTRO_FEATURES += "obmc-ubi-fs"
+DISTROOVERRIDES .= ":df-obmc-ubi-fs"