build-sys: use a separate slirp-obj-y && slirp.mo

This will allow to have cflags for the whole slirp.mo -objs.
It makes it possible to build tests that links only with
slirp-obj-y (and not the whole common-obj).

It is also a step towards building slirp as a shared library, although
this requires a bit more thoughts to build with
net/slirp.o (CONFIG_SLIRP would need to be 'm') and other build issues.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
diff --git a/slirp/Makefile.objs b/slirp/Makefile.objs
index 28049b0..21653f6 100644
--- a/slirp/Makefile.objs
+++ b/slirp/Makefile.objs
@@ -1,5 +1,30 @@
-common-obj-y = cksum.o if.o ip_icmp.o ip6_icmp.o ip6_input.o ip6_output.o \
-               ip_input.o ip_output.o dnssearch.o dhcpv6.o
-common-obj-y += slirp.o mbuf.o misc.o sbuf.o socket.o tcp_input.o tcp_output.o
-common-obj-y += tcp_subr.o tcp_timer.o udp.o udp6.o bootp.o tftp.o arp_table.o \
-                ndp_table.o ncsi.o
+slirp-obj-y = slirp.mo
+
+slirp.mo-objs = \
+	arp_table.o \
+	bootp.o \
+	cksum.o \
+	dhcpv6.o \
+	dnssearch.o \
+	if.o \
+	ip6_icmp.o \
+	ip6_input.o \
+	ip6_output.o \
+	ip_icmp.o \
+	ip_input.o \
+	ip_output.o \
+	mbuf.o \
+	misc.o \
+	ncsi.o \
+	ndp_table.o \
+	sbuf.o \
+	slirp.o \
+	socket.o \
+	tcp_input.o \
+	tcp_output.o \
+	tcp_subr.o \
+	tcp_timer.o \
+	tftp.o \
+	udp.o \
+	udp6.o \
+	$(NULL)