add script to execute gradlew

You can execute it in a subdirectory and it will find and execute
gradlew for the root project.
add coregen tasks for single-pendulum-vhdl
remove generated files in ipcore_dir
extract bitstream compile tasks

I want to use them for single-pendulum-vhdl, so I converted them
to task types.
add bitstream task for single-pendulum-simple
extract download bitstream task, as well
add clean tasks for single-pendulum-vhdl
add new single-pendulum-test tasks to root build script