[handover] Implement handover control logic

Code to implement handover control logic.  A yet-to-be-implemented
handover algorithm will call bsc_handover_start(old_lchan, new_bts)
to start the handover process.
diff --git a/openbsc/src/Makefile.am b/openbsc/src/Makefile.am
index 9c8fa7b..4d18692 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -7,7 +7,7 @@
 noinst_HEADERS = vty/cardshell.h
 
 libbsc_a_SOURCES = abis_rsl.c abis_nm.c gsm_data.c gsm_04_08_utils.c \
-		msgb.c select.c chan_alloc.c timer.c debug.c \
+		msgb.c select.c chan_alloc.c timer.c debug.c handover_logic.c \
 		gsm_subscriber_base.c subchan_demux.c bsc_rll.c transaction.c \
 		trau_frame.c trau_mux.c paging.c e1_config.c e1_input.c tlv_parser.c \
 		input/misdn.c input/ipaccess.c signal.c gsm_utils.c talloc.c \