I tried to build the kernel with the -o option for build output directory in the following way:
./nvbuild.sh -o kernel/kernel-4.9/build_out/
However there is nothing in the build_out dir and though compilation went through, towards end I got this error:
Error: Missing kernel image kernel/kernel-4.9/build_out//arch/arm64/boot/Image
So the -o option does not work as expected. I corrected it by running ./nvbuild.sh again without any option.