bootrom: wait for Vcc to settle before talking to SD card.

If we start talking to the SD card straight from reset we can fail to
send and receive commands.  Wait for 100ms to give it plenty of time to
stabilise.
de0nano/debug: add a reset button.

Use KEY0 on the de0nano board as a reset pin to trigger an external
reset.
bootrom: fix naming of sdcard booting.

This was initially from test code for reading from SD cards.  Fix the
naming to make it more relevant to booting from an SD card.