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/classes/phosphor-software-manager-mmc.bbclass b/meta-phosphor/classes/phosphor-software-manager-mmc.bbclass
index 479619c..0590a83 100644
--- a/meta-phosphor/classes/phosphor-software-manager-mmc.bbclass
+++ b/meta-phosphor/classes/phosphor-software-manager-mmc.bbclass
@@ -2,9 +2,11 @@
 
 PACKAGECONFIG:append = " mmc_layout"
 
-EXTRA_OEMESON += "-Dactive-bmc-max-allowed=2"
-EXTRA_OEMESON += "-Dmedia-dir='/media'"
-EXTRA_OEMESON += "-Doptional-images='image-hostfw'"
+EXTRA_OEMESON:append = " \
+    -Dactive-bmc-max-allowed=2 \
+    -Dmedia-dir='/media' \
+    -Doptional-images='image-hostfw' \
+"
 
 RDEPENDS:phosphor-software-manager-updater-mmc += " \
     gptfdisk \