4-bit SD Card mode

[Warner Losh]

I'm not sure I like this at all.  The devices shouldn't be setting up
the pins for that device.  In some cases, they can't know how to do
that, and this is one of them.  The device has no way of knowing if
all 4 wires are connected, or just 1 and the 'spare' lines are used
for GPIO.  There are similar problems with the serial ports and the
ethernet.

Does the boot loader you are using at least store the board ID in the
place that Linux expects it?

Warner