Should bionic headers be updated when porting from stock kernel?

[gabry3795]

I'm doing a porting from a stock rom to android aosp but when the build system compiles the library libion.so and the device starts up it complains about some missing methods that are present in the /include folder of the stock kernel and so in the kernel headers that the vendor provides!
My question is: how can I compile, for example, the libion.so with the ion.h ion.c provided in stock kernel? How can I tell to the build system to use the vendor header from the stock kernel? I noticed that bionic in libc dir has a tool to update the headers as explained here but when I try to do

bionic/libc/kernel/tools/generate_uapi_headers.sh --download-kernel

it downloads a new kernel from the web? How can I force to use mine in kernel/<vendor>/<device> folder? Do you know any book or site about the relationship between android kernel compiling?

Thanks to someone can answer to me, maybe that's a noob question..