CIAA-ACC: SD card detect signal is necessary for booting from SD.

[nscotti]

In the 1.0 schematic version the SD card detect signal was not considered.
Although some boards boot fine from SD, others don't boot (perhaps because of different chip versions). In the future schematic versions it is recommended to connect the MIO_0 pin to SD card detect signal.
https://www.xilinx.com/support/answers/52016.html
https://www.xilinx.com/support/answers/61064.html
https://www.xilinx.com/support/answers/59476.html