debian: Initial debian packaging

Add initial debian package for plain osmo-pcu (without the
sysmoBTS supporot).
diff --git a/debian/source/format b/debian/source/format
new file mode 100644
index 0000000..89ae9db
--- /dev/null
+++ b/debian/source/format
@@ -0,0 +1 @@
+3.0 (native)