[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_vty.c b/openbsc/src/bsc_vty.c
index 9709301..739d9aa 100644
--- a/openbsc/src/bsc_vty.c
+++ b/openbsc/src/bsc_vty.c
@@ -2702,6 +2702,7 @@
 	install_element(ENABLE_NODE, &pdch_act_cmd);
 
 	abis_nm_vty_init();
+	e1inp_vty_init();
 
 	bsc_vty_init_extra();