Store GPRS MOs directly under BTS SiteMgr object

The only real 1-1 relationship between BTS NM objects is the one between
GPRS Cell and BTS (which is actually a BTS cell).
In our current osmo-bts implementation we don't care much since we only
handle 1-cell BTSses, but let's make the data structure organization
more generic.

Implementation notes:
The gsm_bts_sm is moved to its own file, APIs to allocate are added and
the new public object is hooked correctly in the allocation process of
osmo-bsc.

Change-Id: I06461b7784fa2a78de37383406e35beae85fbad8
diff --git a/tests/gsm0408/Makefile.am b/tests/gsm0408/Makefile.am
index de38747..c04a5ae 100644
--- a/tests/gsm0408/Makefile.am
+++ b/tests/gsm0408/Makefile.am
@@ -29,6 +29,7 @@
 	$(top_builddir)/src/osmo-bsc/arfcn_range_encode.o \
 	$(top_builddir)/src/osmo-bsc/bts.o \
 	$(top_builddir)/src/osmo-bsc/bts_ipaccess_nanobts_omlattr.o \
+	$(top_builddir)/src/osmo-bsc/bts_sm.o \
 	$(top_builddir)/src/osmo-bsc/bts_trx.o \
 	$(top_builddir)/src/osmo-bsc/gsm_data.o \
 	$(top_builddir)/src/osmo-bsc/net_init.o \