meta-phosphor: switch EXTRA_OEMESON to use override syntax

It was observed that we have different approaches to modifying the
EXTRA_OEMESON variable across our recipes.  The bitbake manual
implies that override syntax (":append") is slightly better and the
poky meson bbclass also prefers it:

    meta/classes/meson.bbclass:EXTRA_OEMESON:append = " ${PACKAGECONFIG_CONFARGS}"

Switch all recipes to use the ":append" override syntax for the
EXTRA_OEMESON variable.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I24e9a27a87c61f374acfb03a63955821fce0dd20
diff --git a/meta-phosphor/recipes-extended/sdbusplus/sdbusplus_git.bb b/meta-phosphor/recipes-extended/sdbusplus/sdbusplus_git.bb
index 208b4f3..b4b1019 100644
--- a/meta-phosphor/recipes-extended/sdbusplus/sdbusplus_git.bb
+++ b/meta-phosphor/recipes-extended/sdbusplus/sdbusplus_git.bb
@@ -18,7 +18,7 @@
         systemd \
         "
 
-EXTRA_OEMESON += " \
+EXTRA_OEMESON:append = " \
         -Dtests=disabled \
         -Dexamples=disabled \
         "