meta-ibm: fix bitbake warnings in bmcweb

```
WARNING: EXTRA_OEMESON:append:p10bmc += is not a recommended operator combination, please replace it.
WARNING: EXTRA_OEMESON:append:witherspoon-tacoma += is not a recommended operator combination, please replace it.
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5dc503b322d6cb90feb6d6261802fd50ebca31b2
diff --git a/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend b/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
index 840469b..2ea5d19 100644
--- a/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
+++ b/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
@@ -6,13 +6,13 @@
     -Dredfish-oem-manager-fan-data=disabled \
 "
 
-EXTRA_OEMESON:append:p10bmc += " \
+EXTRA_OEMESON:append:p10bmc = " \
     -Dmutual-tls-auth=disabled \
     -Dkvm=disabled \
     -Dvm-websocket=disabled \
 "
 
-EXTRA_OEMESON:append:witherspoon-tacoma += " \
+EXTRA_OEMESON:append:witherspoon-tacoma = " \
     -Dmutual-tls-auth=disabled \
     -Dkvm=disabled \
     -Dvm-websocket=disabled \