This document is based on the code of android-4.1.1_r6 (Jelly Bean).
# cd $ANDROID_SRC_DIR
# . build/envsetup.sh
# set_stuff_for_environment
# mkdir kernel
# cd kernel
# git clone https://android.googlesource.com/kernel/goldfish.git
# cd goldfish
# $ANDROID_SRC_DIR/external/qemu/distrib/build-kernel.sh --armv7 --out=$ANDROID_SRC_DIR/prebuilt/android-arm/kernel --config=goldfish_armv7
# emulator -kernel prebuilt/android-arm/kernel/kernel-qemu-armv7