kernal.s: roughly extracted splashScreen subroutine.
kernal.s: roughly extracted displayText.
kernal.s: display SD data from two different blocks.

$00000000 is first 512-byte block on card.
$00000200 is second 512-byte block on card.

This proves that multiple blocks can be read from SD; i.e. reading a
block doesn't leave sd.s in a broken state.
sd: SdCardRead reads full 512-byte block.

Previously only 256 bytes were read, leaving it in a broken state;
the card was expecting to send more data, the code in sd.s was expecting
to be able to send the next command.