Move struct gsm_bts: gsm_data.* => bts.*

Place all code related to the object into the related file.

Having all the data model in one file made sense in early stage of
development to make progress quickly, but nowadays it hurts more than
helps, due to constantly growing size and more and more bits being
added to the model, gaining in complexity.

Currently, having lots of different objects mixed up in gsm_data.h is a hole
of despair, where nobody can make any sense were to properly put new stuff
in, ending up with functions related to same object in different files
or with wrong prefixes, declarations of non-existing functions, etc.
because people cannot make up their mind on strict relation to objects
in the data model.
Splitting them in files really helps finding code operating on a
specific object and helping with logically splitting in the future.

Change-Id: I00c15f5285b5c1a0109279b7ab192d5467a04ece
diff --git a/tests/abis/Makefile.am b/tests/abis/Makefile.am
index 60054d9..a01f56a 100644
--- a/tests/abis/Makefile.am
+++ b/tests/abis/Makefile.am
@@ -26,6 +26,7 @@
 
 abis_test_LDADD = \
 	$(top_builddir)/src/osmo-bsc/abis_nm.o \
+	$(top_builddir)/src/osmo-bsc/bts.o \
 	$(top_builddir)/src/osmo-bsc/gsm_data.o \
 	$(top_builddir)/src/osmo-bsc/net_init.o \
 	$(LIBOSMOCORE_LIBS) \
diff --git a/tests/abis/abis_test.c b/tests/abis/abis_test.c
index 43934f3..767a9fb 100644
--- a/tests/abis/abis_test.c
+++ b/tests/abis/abis_test.c
@@ -26,6 +26,7 @@
 #include <osmocom/gsm/gsm23003.h>
 
 #include <osmocom/bsc/gsm_data.h>
+#include <osmocom/bsc/bts.h>
 #include <osmocom/bsc/abis_nm.h>
 #include <osmocom/bsc/debug.h>