if you need to build kernel, you need to use toolchian, mfgtools is for download image to your board,you can refer to the document as below:
Task #8 - Build kernel manually using created toolchain
for path, you can find the dts file under:
fsl-release-bsp/build/tmp/work-shared/imx6qsabresd/kernel-source/arch/arm/boot/dts