Compiling with android ndk

export NDK_PATH=</work/ndk>
export TOOLCHAIN=/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc
"${NDK_PATH}${TOOLCHAIN}" -Os bootmenu.c -o bootmenu -static -Wl,--gc-sections -Wl,--strip-all --sysroot=$NDK_PATH/platforms/android-13/arch-arm/

Unpacking / repacking a boot.img

Extract boot.img

abootimg -x boot.img

Extract initrd.img

mkdir initrd
cd initrd
cat ../initrd.img | gunzip | cpio -vid

Package initrd.img

find . | cpio --create --format='newc' | gzip > ../initrd_new.img

Package boot.img

abootimg --create boot_new.img -f bootimg.cfg -k zImage -r initrd_new.img