filter: Move the gsm 04.08 filter to a common place

For customer requirements we want to be able to do
filtering on the BSC as well. The same messages need
to be scanned and the same access-lists will be looked
at. In the future we might even split traffic based
on the IMSI. Begin with moving the code to a new top
level directory and then renaming and removing the
nat dependency.
diff --git a/openbsc/tests/bsc-nat/Makefile.am b/openbsc/tests/bsc-nat/Makefile.am
index b620d96..64cd9ac 100644
--- a/openbsc/tests/bsc-nat/Makefile.am
+++ b/openbsc/tests/bsc-nat/Makefile.am
@@ -10,11 +10,12 @@
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_filter.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_sccp.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_utils.c \
-			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_filter.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_rewrite.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_rewrite_trie.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_mgcp_utils.c
-bsc_nat_test_LDADD = $(top_builddir)/src/libbsc/libbsc.a \
+bsc_nat_test_LDADD = \
+			$(top_builddir)/src/libfilter/libfilter.a \
+			$(top_builddir)/src/libbsc/libbsc.a \
 			$(top_builddir)/src/libmgcp/libmgcp.a \
 			$(top_builddir)/src/libtrau/libtrau.a \
 			$(top_builddir)/src/libcommon/libcommon.a \