Add SD-Card change and SD-Card init. for electronics equipped with LCD Display

Some reprap electronics with limited µcomtroller pins e.g. the Saguinololu do not implement the SD-card change detect pin. In this case the user has to press RESET to detect the SD card. For electronics equipped with a LCD and encoder wheel it would be possible to do this manually via the menu without resetting the electronics.

This pull request implements in the menu the detection and change of the SD-card.