pcu_l1_if_phy: support multiple BTS (in theory)

The PCU already has a list that can hold multiple BTS objects but the
API for the direct PHY access has no way to associate a PHY with a
certain BTS object. Lets update the API so that we can associate a BTS
object when opening a PDCH.

Unfortunately OsmoPCU has never been tested with more than one BTS so it
is very likely that there are still shortcomings that prevent OsmoPCU to
work properly when more then one BTS is attached. To make users aware of
this, also print a warning as soon as more than one BTS object exists.

Related: OS#5198
Related: OS#5930
Change-Id: I33518cbbe83f7f8c071afa5e995d30930099ba92
diff --git a/src/osmo-bts-sysmo/sysmo_l1_if.c b/src/osmo-bts-sysmo/sysmo_l1_if.c
index 348497f..99ccf9a 100644
--- a/src/osmo-bts-sysmo/sysmo_l1_if.c
+++ b/src/osmo-bts-sysmo/sysmo_l1_if.c
@@ -352,7 +352,7 @@
 	return 0;
 }
 
-void *l1if_open_pdch(uint8_t trx_no, uint32_t hlayer1, struct gsmtap_inst *gsmtap)
+void *l1if_open_pdch(uint8_t bts_nr, uint8_t trx_no, uint32_t hlayer1, struct gsmtap_inst *gsmtap)
 {
 	struct femtol1_hdl *fl1h;
 	int rc;