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/regenerate-meson b/gen/regenerate-meson
new file mode 100755
index 0000000..b9cb192
--- /dev/null
+++ b/gen/regenerate-meson
@@ -0,0 +1,3 @@
+#!/bin/sh
+${SDBUSPP_GEN_MESON:=../subprojects/sdbusplus/tools/sdbus++-gen-meson} \
+    --directory .. --command meson --output .