Fix bmcweb append in openpower

Tested:
Booted up Romulus in QEMU and verified the following:
- bmcweb started without error
- Redfish command to view dbus event logs worked

Change-Id: I1b8b6b76600c82233f00e3cf19d513cdf7e3a9e1
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/recipes-phosphor/bmcweb/bmcweb_%.bbappend b/recipes-phosphor/bmcweb/bmcweb_%.bbappend
index 0765ddf..1035bf1 100644
--- a/recipes-phosphor/bmcweb/bmcweb_%.bbappend
+++ b/recipes-phosphor/bmcweb/bmcweb_%.bbappend
@@ -1 +1 @@
-EXTRA_OECMAKE_df-openpower += "-DBMCWEB_ENABLE_REDFISH_DBUS_LOG_ENTRIES=ON"
+EXTRA_OECMAKE_append_df-openpower = " -DBMCWEB_ENABLE_REDFISH_DBUS_LOG_ENTRIES=ON"