s1gw: initial osmo-s1gw testsuite skeleton

Change-Id: If0521cbfe9576c2529bf13579899bc64a50e45b3
Related: SYS#6772
diff --git a/Makefile b/Makefile
index d216aab..5a35e6e 100644
--- a/Makefile
+++ b/Makefile
@@ -39,6 +39,7 @@
 	pcu \
 	pgw \
 	remsim \
+	s1gw \
 	sccp \
 	selftest \
 	sgsn \