use pcb (not demo board) pins for sd chip select
write a frame of data to fpga's ram

simpler than to external ram.
(no address required. just shift and flip.)

reads a single frame from sd card and writes to fpga
use correct pins for clear shift and flip
rename sd_card to sd_block_device
create sd frame source
loop forever reading and writing frames
return duration and size at same time as data

get rid of the desc stuff, just return those values at same time as
returning data. caller must have a buffer big enough for maximum
frame size.