Export BUILD_DIR which points to the current build directory

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Add help message in Makefile

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Allow custom build/install/uninstall scripts

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Include custom settings on what boards are supported

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>