Turned out to be pretty easy with all the more complex bits
already in place.
Strategy is to always parse a full line from one of the sources.
Accordingly, simply sending a character on the serial line stops
reading from SD until the line coming in over serial is completed.
Next to the implementation of sd_read_byte() as well as M24 and
M25, yet another demo: read the file and write it to the serial
line, to show correctness of the implementation.
Markus Hitter
Michael Moon