treewide: Create yaml directory

This allows the propagated yamldir paramter to work correctly in
subproject style builds. We don't want other files in the project
conflicting with tools that scan the yamldir.

Change-Id: Ie2759c7f3baf399392cdda0e507055ceb262d2e1
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/gen/regenerate-meson b/gen/regenerate-meson
index 56bc2c6..a39ad64 100755
--- a/gen/regenerate-meson
+++ b/gen/regenerate-meson
@@ -19,5 +19,5 @@
 
 $SDBUSPP_GEN_MESON \
     --command meson \
-    --directory "$REPO_ROOT" \
+    --directory "$REPO_ROOT/yaml" \
     --output "$REPO_ROOT/gen"