aufs-util: Convert confusing append to override syntax

While in this case `RRECOMMENDS:class-target +=` wouldn't result in any
unwanted override, there is no guarantee there won't be a change, which
would be hidden by this override. To avoid any surprises in the future
let's use `:append:class-target =` syntax here.

Signed-off-by: Michal Sieron <michalwsieron@gmail.com>
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-filesystems/recipes-utils/aufs-util/aufs-util_git.bb

@@ -63,6 +63,6 @@ do_install:class-native () {
     done
 }
 
-RRECOMMENDS:${PN}:class-target += "kernel-module-aufs"
+RRECOMMENDS:${PN}:append:class-target = " kernel-module-aufs"
 
 BBCLASSEXTEND = "native"