TinyUSB examples includes external repos aka submodules to provide low-level MCU peripheral's driver to compile with. Therefore we will firstly fetch those mcu driver repo by running this command in the top folder repo
`flash` target will use the on-board debugger (jlink/cmsisdap/stlink/dfu) to flash the binary. We should install those debugger/programmer software in advance. Futhermore, since external jlink can be used with most of the board, there is also `flash-jlink` target for out convenience.