SGSN: prevent starting with inconsistent config

Previously it was possible to start osmo-sgsn with "auth-policy remote"
but without "gsup remote-*" which resulted in broken setup: no MS could
perform GPRS ATTACH. Add consistency check to vty code to fix this.

Related: OS#1582
Change-Id: Ie4296e7d99d7833f7d828b0196435ea81097cf6e
diff --git a/debian/changelog b/debian/changelog
index 6a0362a..e9a4212 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,6 +1,8 @@
 openbsc (0.15.1) UNRELEASED; urgency=medium
 
   * Move forward toward a new release.
+  * Prevent SGSN starting with 'auth-policy remote' when no 'gsup remote-*' are configured.
+    Note: such configs are broken without extra workarounds anyway.
 
  -- Holger Hans Peter Freyther <holger@moiji-mobile.com>  Tue, 24 May 2016 23:14:31 +0200