move log/debug codebase to libosmocore

The logging/debugging code is generic enough to move it into libosmocore
while keeping OpenBSC specific definitions in openbsc itself.

This commit uses the logging support present in libosmocore-0.1.2,
you will have to update your library to this version.
diff --git a/openbsc/src/telnet_interface.c b/openbsc/src/telnet_interface.c
index aa119b4..c7de026 100644
--- a/openbsc/src/telnet_interface.c
+++ b/openbsc/src/telnet_interface.c
@@ -117,7 +117,7 @@
 	bsc_unregister_fd(fd);
 
 	if (conn->dbg) {
-		debug_del_target(conn->dbg);
+		log_del_target(conn->dbg);
 		talloc_free(conn->dbg);
 	}