[HSL] initial support for the HSL 2.75G Femtocell

The HSL Femtocell seems to be a poor man implementation of the
ip.access Abis/IP protocol, but cutting corners wherever possible.

We try to workaround those corners wherever possible...
diff --git a/openbsc/src/libabis/Makefile.am b/openbsc/src/libabis/Makefile.am
index 7f5ac47..0df7b5a 100644
--- a/openbsc/src/libabis/Makefile.am
+++ b/openbsc/src/libabis/Makefile.am
@@ -7,6 +7,7 @@
 libabis_a_SOURCES = e1_input.c e1_input_vty.c \
 			input/misdn.c		\
 			input/ipaccess.c	\
+			input/hsl.c		\
 			input/dahdi.c		\
 			input/lapd.c