[BSC] Move the BTS-type specific code from bcs_init.c to bts_*.c

bsc_init.c was a big mess even only for two supported BTS models,
so before adding more BTS types, this needs a cleanup.

All the BTS specific code from bsc_init.c has now moved into
bts_{siemens_bs11,ipaccess_nanobts}.c

This has required that input_event() and nm_state_event() get both
converted to proper libosmocore signals instead of referencing external
symbols.
diff --git a/openbsc/tests/channel/channel_test.c b/openbsc/tests/channel/channel_test.c
index 8403997..4f3c593 100644
--- a/openbsc/tests/channel/channel_test.c
+++ b/openbsc/tests/channel/channel_test.c
@@ -72,8 +72,7 @@
 	}
 }
 
-void nm_state_event() {}
-void input_event() {}
+void _abis_nm_sendmsg() {}
 void sms_alloc() {}
 void gsm_net_update_ctype(struct gsm_network *network) {}
 void gsm48_secure_channel() {}