build: add sdbus++-gen-meson generated files

Use the tool to generate the meson.build files for the current
tree of interfaces, but do not utilize them yet.  Add a
small helper tool to regenerate as needed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie1f68984e5b01dcab661863d2ac719d0e3cd45c1
diff --git a/gen/org/meson.build b/gen/org/meson.build
new file mode 100644
index 0000000..6488f22
--- /dev/null
+++ b/gen/org/meson.build
@@ -0,0 +1,2 @@
+# Generated file; do not modify.
+subdir('open_power')