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/common/recipes-kernel/linux/linux-obmc.inc b/meta-phosphor/common/recipes-kernel/linux/linux-obmc.inc
index 72c2334..9797811 100644
--- a/meta-phosphor/common/recipes-kernel/linux/linux-obmc.inc
+++ b/meta-phosphor/common/recipes-kernel/linux/linux-obmc.inc
@@ -14,7 +14,7 @@
 SRC_URI += "file://phosphor-gpio-keys.cfg"
 SRC_URI += "file://phosphor-vlan.scc"
 SRC_URI += "file://phosphor-vlan.cfg"
-SRC_URI += "${@mf_enabled(d, 'obmc-ubi-fs', 'file://0001-ARM-dts-Aspeed-Witherspoon-Update-BMC-partitioning.patch')}"
+SRC_URI_append_df-obmc-ubi-fs = " file://0001-ARM-dts-Aspeed-Witherspoon-Update-BMC-partitioning.patch"
 
 
 LINUX_VERSION_EXTENSION ?= "-${SRCREV}"