VPD Error interface UnknownSystemType supported

DevTree gets selected based on HW and IM of the system VPD.
If we get anything unexpected from these keywords, it couldn't
determine which systemType it is and so shouldn't load DevTree.
A PEL with this error interface UnknownSystemType will be logged
and break the booting process there itself.

Change-Id: I0437f4db87b4ab556d7c5433a861689c682fafe4
Signed-off-by: Giridhari Krishna <giridharikrishnan@gmail.com>
diff --git a/yaml/com/ibm/VPD.errors.yaml b/yaml/com/ibm/VPD.errors.yaml
index da408f0..514a449 100644
--- a/yaml/com/ibm/VPD.errors.yaml
+++ b/yaml/com/ibm/VPD.errors.yaml
@@ -23,3 +23,8 @@
   description: Invalid Json file.
 - name: DbusFailure
   description: DBus error occurred.
+- name: UnknownSystemType
+  description:
+      Could not determine which device tree to switch to, as system type is
+      unknown. Please check the HW and  IM keywords in the system VPD and check
+      if they are programmed correctly.