build: Move yaml files into a subdirectory

This improves generation speed and ensures that the yamldir only
contains yaml files.

Change-Id: I6094f23dfc4af2bb43ca7fdba047014142e1991f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/gen/regenerate-meson b/gen/regenerate-meson
index 5b4fc06..e2a02c4 100755
--- a/gen/regenerate-meson
+++ b/gen/regenerate-meson
@@ -1,4 +1,4 @@
 #!/bin/bash
 cd "$(dirname "$0")" || exit
 export PATH="$PWD/../subprojects/sdbusplus/tools:$PATH"
-exec sdbus++-gen-meson --command meson --directory .. --output .
+exec sdbus++-gen-meson --command meson --directory ../yaml --output .