input: add rs232 driver for virtual E1 lines

This patch adds the rs232 driver which is used by then bs11_config
utility available in openBSC.
diff --git a/src/input/Makefile.am b/src/input/Makefile.am
index c382269..9937465 100644
--- a/src/input/Makefile.am
+++ b/src/input/Makefile.am
@@ -9,4 +9,5 @@
 				ipa.c		\
 				ipaccess.c	\
 				lapd.c		\
-				misdn.c
+				misdn.c		\
+				rs232.c