RSL: restructure MDCX functions

In preparation for upcoming LCLS changes we have to split IPA RSL MDCX
generation into separate function with the ability to set destination
explicitly instead of just using the value from lchan which will be used
in follow-up patches.

Change-Id: Iffe2f4f10e841fc36965cce02b4e5f017a5ae6c8
Related: OS#3659
diff --git a/include/osmocom/bsc/abis_rsl.h b/include/osmocom/bsc/abis_rsl.h
index 5ada3fc..098d2e6 100644
--- a/include/osmocom/bsc/abis_rsl.h
+++ b/include/osmocom/bsc/abis_rsl.h
@@ -61,6 +61,7 @@
 int rsl_siemens_mrpci(struct gsm_lchan *lchan, struct rsl_mrpci *mrpci);
 
 /* ip.access specfic RSL extensions */
+struct msgb *rsl_make_ipacc_mdcx(const struct gsm_lchan *lchan, uint32_t dest_ip, uint16_t dest_port);
 int rsl_tx_ipacc_crcx(const struct gsm_lchan *lchan);
 int rsl_tx_ipacc_mdcx(const struct gsm_lchan *lchan);
 int rsl_ipacc_mdcx_to_rtpsock(struct gsm_lchan *lchan);