add support to build netmap as a module,

but for the time being keep it disconnected from the main build.

MFC after:	3 days

sys/modules/netmap/Makefile

@@ -0,0 +1,14 @@
+# $FreeBSD$
+#
+# Compile netmap as a module, useful if you want a netmap bridge
+# or loadable drivers.
+
+.PATH: ${.CURDIR}/../../dev/netmap
+.PATH.h: ${.CURDIR}/../../net
+KMOD	= netmap
+SRCS	= device_if.h bus_if.h opt_netmap.h
+SRCS	+= netmap.c netmap.h netmap_kern.h
+
+netmap.o: netmap_mem2.c
+
+.include <bsd.kmod.mk>