In this directory, you must define such as makefiles:
BoardConfig.mk: include by build/core/envsetup.mk
YudatunBoard.mk: include by build/target/board/Yudatun.mk
yudatun_qemu.mk: include by build/target/product/YudatunProducts.mk
In other words, you just to define above three makefiles, you can DIY yourself hardware.