re-structure the OpenBSC directory layout

The new structure divides the code into a number of libraries
for the BSC core functionality, MSC core functionality, Abis transport,
TRAU and other bits.

This doesn't introduce any functional code change but simply moves
around files and alters Makefile.am accordingly.

Next step would be to disentangle a lot of the inter-library
dependencies and make the individual bits of code more independent.
diff --git a/openbsc/configure.in b/openbsc/configure.in
index 50f7dbc..525bad8 100644
--- a/openbsc/configure.in
+++ b/openbsc/configure.in
@@ -87,10 +87,18 @@
     include/openbsc/Makefile
     include/Makefile
     src/Makefile
-    src/ipaccess/Makefile
-    src/gprs/Makefile
-    src/nat/Makefile
+    src/trau/Makefile
+    src/abis/Makefile
     src/bsc/Makefile
+    src/msc/Makefile
+    src/mgcp/Makefile
+    src/common/Makefile
+    src/osmo-nitb/Makefile
+    src/osmo-bsc/Makefile
+    src/nat/Makefile
+    src/ipaccess/Makefile
+    src/utils/Makefile
+    src/gprs/Makefile
     tests/Makefile
     tests/debug/Makefile
     tests/gsm0408/Makefile