meta-ibm: use ibm-ac-server MACHINEOVERRIDE

Use an override in bbappends in meta-witherspoon as described and
recommended in the "following best practices when creating layers"
section of the the Yocto development tasks manual.

ibm-ac-server represents common machine configuration shared by all of
IBMs "Accelerated Compute" class of servers.  Apply it wherever any
variables are un-bounded by any override.

This enables more flexible use of meta-witherspoon in build
configurations, and easy addition of machines to the layer.

(From meta-ibm rev: bb0e1d0699a4b9905c053c94af7d882cd71779fc)

Change-Id: I82f7338b0d7a64b5e7a67e19bcdff07f48a332d4
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-ibm/meta-witherspoon/recipes-phosphor/flash/openpower-software-manager_%.bbappend b/meta-ibm/meta-witherspoon/recipes-phosphor/flash/openpower-software-manager_%.bbappend
index 3dcc25d..197e404 100644
--- a/meta-ibm/meta-witherspoon/recipes-phosphor/flash/openpower-software-manager_%.bbappend
+++ b/meta-ibm/meta-witherspoon/recipes-phosphor/flash/openpower-software-manager_%.bbappend
@@ -1 +1 @@
-PACKAGECONFIG_append_df-openpower-ubi-fs = " verify_pnor_signature"
+PACKAGECONFIG_append_ibm-ac-server = " verify_pnor_signature"