Add simplistic telnet control interface

This might turn into a complete wire protocol with special
client software. For now it will be a simple client interface
that you can use with telnet to do certain things.

This is using flex to implement the parsing. Implementation
and more commands will follow.
diff --git a/include/openbsc/Makefile.am b/include/openbsc/Makefile.am
index c1c3dd6..f1f163b 100644
--- a/include/openbsc/Makefile.am
+++ b/include/openbsc/Makefile.am
@@ -1,3 +1,3 @@
 noinst_HEADERS = abis_nm.h abis_rsl.h debug.h db.h gsm_04_08.h gsm_data.h \
 		 gsm_subscriber.h linuxlist.h msgb.h select.h tlv.h gsm_04_11.h \
-		 timer.h misdn.h chan_alloc.h
+		 timer.h misdn.h chan_alloc.h telnet_interface.h