[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/bsc_hack.c b/openbsc/src/bsc_hack.c
index 11499ab..dacaad3 100644
--- a/openbsc/src/bsc_hack.c
+++ b/openbsc/src/bsc_hack.c
@@ -238,6 +238,8 @@
 	bts_model_bs11_init();
 	bts_model_nanobts_init();
 
+	e1inp_init();
+
 	/* enable filters */
 	log_set_all_filter(stderr_target, 1);