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-phosphor/state/phosphor-state-manager_git.bb b/meta-phosphor/recipes-phosphor/state/phosphor-state-manager_git.bb
index 6b527c3..83939c4 100644
--- a/meta-phosphor/recipes-phosphor/state/phosphor-state-manager_git.bb
+++ b/meta-phosphor/recipes-phosphor/state/phosphor-state-manager_git.bb
@@ -57,7 +57,7 @@
 
 RDEPENDS:${PN}-chassis += "bash"
 
-EXTRA_OEMESON += "-Dtests=disabled"
+EXTRA_OEMESON:append = " -Dtests=disabled"
 
 FILES:${PN}-host = "${bindir}/phosphor-host-state-manager"
 DBUS_SERVICE:${PN}-host += "xyz.openbmc_project.State.Host.service"