meta-ibm: add ibm-ac-server MACHINEOVERRIDE

As of today the IBM(witherspoon) layer supports three of IBMs
"Acclerated Compute" class of systems: witherspoon, witherspoon-128 and
swift.

Most of the bbappends are required for all three.  Prior to this patch
this was handled by bbappending without any override.  This is counter
to best practices and pollutes the metadata for layer users using
recipes from meta-witherspoon.

Add a new ibm-ac-server MACHINEOVERRIDE so that bbappends that work on
all of IBM's AC systems can be overridden appropriately without having
to explicitly list every system.

(From meta-ibm rev: b770d19fc3e9774a1ad80be55b2f986d02847c26)

Change-Id: I69ae41fa80612d1c019f031a02e32c01580b7939
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-ibm/meta-witherspoon/conf/machine/witherspoon.conf b/meta-ibm/meta-witherspoon/conf/machine/witherspoon.conf
index 579b00d..3f7aedc 100644
--- a/meta-ibm/meta-witherspoon/conf/machine/witherspoon.conf
+++ b/meta-ibm/meta-witherspoon/conf/machine/witherspoon.conf
@@ -8,6 +8,9 @@
 
 PREFERRED_PROVIDER_virtual/p9-vcs-workaround = ''
 
+# A machine override for common machine configuration for all of IBM's
+# "Accelerated Computing" class of systems.
+MACHINEOVERRIDES =. "ibm-ac-server:"
 require conf/machine/include/ast2500.inc
 require conf/machine/include/obmc-bsp-common.inc
 require conf/machine/include/openpower.inc