picoimx: resolve PRODUCT_OUT late

Change-Id: Ifcedb15221dd053c1523be3414c51cf20929c822
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 121281b..ecd7b87 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -49,5 +49,7 @@
 WIFI_DRIVER_HAL_MODULE := wifi_driver.$(soc_name)
 WIFI_DRIVER_HAL_PERIPHERAL := bcm4339
 
-BOARD_MKBOOTIMG_ARGS := --second $(PRODUCT_OUT)/kernel.dtb \
+# $(PRODUCT_OUT) hasn't been defined yet, so use "=" instead of ":="
+# so that it is resolved late.
+BOARD_MKBOOTIMG_ARGS = --second $(PRODUCT_OUT)/kernel.dtb \
 						--base $(BOARD_KERNEL_BASE)