Writing to floppies

Main update in this release is the addition of SD card write support. This works for the MCU and it is e.g. possible to save the current settings to SD card and it also works for the core which can now write to floppy disk images. Further improvements in this release are the possibility to use a USB joystick and an interrupt signal between the FPGA and the MCU. This mainly allows for faster response times during SD card access.

Together with the cores ability to handle ED floppy disk images with a capacity of nearly 3MB, complex use cases are now supported even without harddisk support (yet).

** Important: This release adds a new IRQ signal between the FPGA and the MCU. In order for the SD card to work properly, PIn IO14 of the M0S needs to be connected to pin 51 of the Tang Nano 20k.**