Blackfin: move watchdog config check to Makefile

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/cpu/blackfin/Makefile b/cpu/blackfin/Makefile
index 5eef6a3..211b8d5 100644
--- a/cpu/blackfin/Makefile
+++ b/cpu/blackfin/Makefile
@@ -24,7 +24,7 @@
 COBJS-y  += reset.o
 COBJS-y  += serial.o
 COBJS-y  += traps.o
-COBJS-y  += watchdog.o
+COBJS-$(CONFIG_HW_WATCHDOG)  += watchdog.o
 
 ifeq ($(CONFIG_BFIN_BOOT_MODE),BFIN_BOOT_BYPASS)
 COBJS-y  += initcode.o