rpi-wifi-bridge uses Buildroot, and you can compile it yourself to customize things like the kernel, included packages, and much, much more.
git clone --depth=1 --recurse-submodules --shallow-submodules https://github.com/owenthewizard/rpi-wifi-bridge.git
cd rpi-wifi-bridge
# generate configs for RPi 0W / RPi 2 targets
./gen_configs.sh
# for all targets - untested
./gen_configs.sh --all
cd rpi-wifi-bridge/buildroot
export BR2_EXTERNAL=$PWD/../
# choose your target
make raspberrypi0w_defconfig
# make your customizations
make nconfig
# build
make
# flash to sdcard (replace sdX)
pv -Y output/images/sdcard.img > /dev/sdX
For more information on how to use Buildroot, see the manual.