misc: Introduce a more neutral configure option for CTRL/VTY

We started with only testing the VTY but now test VTY and CTRL
interface with this python framework and might even extend this
to SMPP. So add and "--enable-external-tests" directive which
enables the external interface tests.
diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index a29d9fb..7bb8712 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -35,7 +35,7 @@
 TESTSUITE = $(srcdir)/testsuite
 DISTCLEANFILES = atconfig
 
-if ENABLE_VTY_TESTS
+if ENABLE_EXT_TESTS
 python-tests: $(BUILT_SOURCES)
 	osmotestvty.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v
 	osmotestconfig.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v