Consistenly format variables in */Makefile.am files

Change-Id: Ifa21513c007072314097b7bec188579972dc1694
diff --git a/openbsc/src/libfilter/Makefile.am b/openbsc/src/libfilter/Makefile.am
index ed3cd43..6d3db0b 100644
--- a/openbsc/src/libfilter/Makefile.am
+++ b/openbsc/src/libfilter/Makefile.am
@@ -1,11 +1,26 @@
-AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
-AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) \
-	$(LIBOSMOVTY_CFLAGS) $(LIBOSMOABIS_CFLAGS) $(LIBOSMOSCCP_CFLAGS) $(COVERAGE_CFLAGS)
+AM_CPPFLAGS = \
+	$(all_includes) \
+	-I$(top_srcdir)/include \
+	-I$(top_builddir) \
+	$(NULL)
 
-noinst_LIBRARIES = libfilter.a
+AM_CFLAGS = \
+	-Wall \
+	$(LIBOSMOCORE_CFLAGS) \
+	$(LIBOSMOGSM_CFLAGS) \
+	$(LIBOSMOVTY_CFLAGS) \
+	$(LIBOSMOABIS_CFLAGS) \
+	$(LIBOSMOSCCP_CFLAGS) \
+	$(COVERAGE_CFLAGS) \
+	$(NULL)
+
+noinst_LIBRARIES = \
+	libfilter.a \
+	$(NULL)
 
 libfilter_a_SOURCES = \
 	bsc_msg_filter.c \
 	bsc_msg_acc.c \
-	bsc_msg_vty.c
+	bsc_msg_vty.c \
+	$(NULL)