ctrl: Move the lookup into a separate file in preparation for GPRS

For GPRS the look-up via bts/trx does not make any sense and would
introduce bad depdencies for the SGSN. Move the look-up code to a
new file and introduce new setup methods.
diff --git a/openbsc/src/libbsc/Makefile.am b/openbsc/src/libbsc/Makefile.am
index 5330091..0c22662 100644
--- a/openbsc/src/libbsc/Makefile.am
+++ b/openbsc/src/libbsc/Makefile.am
@@ -22,5 +22,6 @@
 			bsc_api.c bsc_msc.c bsc_vty.c \
 			gsm_04_08_utils.c \
 			bsc_init.c bts_init.c bsc_rf_ctrl.c \
-			arfcn_range_encode.c bsc_ctrl_commands.c
+			arfcn_range_encode.c bsc_ctrl_commands.c \
+			bsc_ctrl_lookup.c