Add new BSC RLL layer of code

A caller can call rll_establish(lchan, link_id) and a callback to the GSM RLL
code.  He will get called back if the RLL link is established or receives some
error message, or the establishment times out.

We need this for proper SMS implementation, where we need to restablish a SAPI3
RLL link before transmitting the actual CP-DATA messages.
diff --git a/openbsc/include/openbsc/abis_rsl.h b/openbsc/include/openbsc/abis_rsl.h
index 02bc521..a8734d1 100644
--- a/openbsc/include/openbsc/abis_rsl.h
+++ b/openbsc/include/openbsc/abis_rsl.h
@@ -476,6 +476,7 @@
 int rsl_imm_assign_cmd(struct gsm_bts *bts, u_int8_t len, u_int8_t *val);
 
 int rsl_data_request(struct msgb *msg, u_int8_t link_id);
+int rsl_establish_request(struct gsm_lchan *lchan, u_int8_t link_id);
 int rsl_relase_request(struct gsm_lchan *lchan, u_int8_t link_id);
 
 /* ip.access specfic RSL extensions */