The device tree in Android is used a secondary image created by mkbootimg with --second.
You can check details in Android official website.
TARGET_BOARD_DTS_CONFIG is the variable for the dtb files.
Hi,
It is ..arch/arm/boot/dts
linux-2.6-imx.git - Freescale i.MX Linux Tree
Hope this can help you.
Have a nice day
Dan