picoimx: use correct output variable

Change-Id: I85a02f6b531078f63e27e362662ead157e437753

BoardConfig.mk

diff --git a/BoardConfig.mk b/BoardConfig.mk
index 9d8ceb2..121281b 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -49,5 +49,5 @@
 WIFI_DRIVER_HAL_MODULE := wifi_driver.$(soc_name)
 WIFI_DRIVER_HAL_PERIPHERAL := bcm4339
 
-BOARD_MKBOOTIMG_ARGS := --second $(ANDROID_PRODUCT_OUT)/kernel.dtb \
+BOARD_MKBOOTIMG_ARGS := --second $(PRODUCT_OUT)/kernel.dtb \
 						--base $(BOARD_KERNEL_BASE)