build_image_util: Speed up build_image with FEATURES="-merge-wait"

I see a 25% improvement. `man make.conf` says that this can potentially
result in build failures, but it seems unlikely when merely installing
binary packages. I haven't had any issues after several local builds.

Signed-off-by: James Le Cuirot <jlecuirot@microsoft.com>

build_library/build_image_util.sh

@@ -150,7 +150,7 @@ emerge_to_image() {
   fi
 
   sudo -E ROOT="${root_fs_dir}" \
-      FEATURES="-ebuild-locks" \
+      FEATURES="-ebuild-locks -merge-wait" \
       PORTAGE_CONFIGROOT="${BUILD_DIR}"/configroot \
       emerge --usepkgonly --jobs="${NUM_JOBS}" --verbose "$@"