xtensa: ISS: only build networking if configured

This allows building kernel in allnoconfig configuration.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/platforms/iss/Makefile

@@ -4,5 +4,5 @@
 # "prom monitor" library routines under Linux.
 #
 
-obj-y			= io.o console.o setup.o network.o
-
+obj-y			= io.o console.o setup.o
+obj-$(CONFIG_NET)	+= network.o