ne2000 driver: change #ifdef to Makefile conditional compilation

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/drivers/net/Makefile b/drivers/net/Makefile
index 321bd08..7d7df9c 100644
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -41,7 +41,7 @@
 COBJS-y += macb.o
 COBJS-y += mcffec.o
 COBJS-y += natsemi.o
-COBJS-y += ne2000.o
+COBJS-$(CONFIG_DRIVER_NE2000) += ne2000.o
 COBJS-y += netarm_eth.o
 COBJS-y += netconsole.o
 COBJS-y += ns7520_eth.o