If you haven't already, build the Android NDK standalone toolchains:

mkdir ~/build/ndk
mkdir ~/build/toolchain
cd ~/build/ndk
wget http://dl.google.com/android/ndk/android-ndk-r10e-linux-x86_64.bin
chmod +x android-ndk-r10e-linux-x86_64.bin
./android-ndk-r10e-linux-x86_64.bin
cd android-ndk-r10e
build/tools/make-standalone-toolchain.sh --platform=android-21 --toolchain=arm-linux-androideabi-4.9 --install-dir="$HOME/build/toolchain/android-armhf-4.9"
build/tools/make-standalone-toolchain.sh --platform=android-21 --toolchain=aarch64-linux-android-4.9 --install-dir="$HOME/build/toolchain/android-arm64-4.9"
build/tools/make-standalone-toolchain.sh --platform=android-21 --toolchain=x86_64-4.9 --install-dir="$HOME/build/toolchain/android-amd64-4.9"
build/tools/make-standalone-toolchain.sh --platform=android-21 --toolchain=x86-4.9 --install-dir="$HOME/build/toolchain/android-i386-4.9"

To properly set up this repository, you must use git clone --recursive.
Alternatively, use git submodule update inside the repository after cloning.

cd ~/build
git clone --recursive https://github.com/jcadduono/nethunter-installer-tools.git

Use ./android through source, ex.
source ./android

If building for TWRP (/system not mounted):
TWRP=1 source ./android

If building for armhf:
ARCH=armhf source ./android

If building for arm64:
ARCH=arm64 source ./android

If building for amd64:
ARCH=amd64 source ./android

If building for i386:
ARCH=i386 source ./android

You can set optimization levels too:
O=3 source android

You can also force static compiled binaries:
STATIC=1 source android

If you would like to build specific projects for each arch with the standard settings, you can use (by itself):
./build_all.sh [project_name1] [project_name2]

To arrange the build output binaries into the same structure used in the Kali NetHunter installer git for easy copy/paste:
./create_structure.sh