[E1 INPUT] explicitly configure the E1 input driver for each line

This introduces a new 'e1_input' config node with a command to be
used like:
  e1_line 0 driver misdn

This allows us to have different input drivers in the future
diff --git a/openbsc/src/Makefile.am b/openbsc/src/Makefile.am
index 290bc06..351b269 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -19,7 +19,8 @@
 libbsc_a_SOURCES = abis_rsl.c abis_nm.c gsm_data.c gsm_04_08_utils.c \
 		chan_alloc.c debug.c socket.c abis_nm_vty.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 \
+		trau_frame.c trau_mux.c paging.c \
+		e1_config.c e1_input.c e1_input_vty.c \
 		input/misdn.c input/ipaccess.c handover_logic.c \
 		talloc_ctx.c system_information.c rest_octets.c \
 		bts_siemens_bs11.c bts_ipaccess_nanobts.c mncc_upqueue.c \