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/README b/gen/README
new file mode 100644
index 0000000..71182b5
--- /dev/null
+++ b/gen/README
@@ -0,0 +1,2 @@
+This directory contains generated meson.build files from sdbus++-gen-meson.
+Do not edit them by hand.