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