hw/nios2/Makefile.objs: Conditionally build nios2

CONFIG_NIOS2_10M50 added for 10m50 dev board.
Signed-off-by: NYang Zhong <yang.zhong@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20190202072456.6468-18-yang.zhong@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

default-configs/nios2-softmmu.mak

@@ -4,3 +4,4 @@ CONFIG_NIOS2=y
 CONFIG_SERIAL=y
 CONFIG_PTIMER=y
 CONFIG_ALTERA_TIMER=y
+CONFIG_NIOS2_10M50=y

hw/nios2/Makefile.objs

-obj-y = boot.o cpu_pic.o 10m50_devboard.o
+obj-y = boot.o cpu_pic.o
+obj-$(CONFIG_NIOS2_10M50) += 10m50_devboard.o