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The previous timeout of 60 seconds was mostly based on scenarios
where large ("write") transfers take place. But it could easily
become annoying if users are awaiting completion of simpler
commands like "read" or "hexdump", and for some reason FEL fails
to respond.

Therefore I've decided to lower the timeout value to 10 seconds,
adjust the maximum chunk size accordingly and - while at it -
improve the source comments documenting their relationship.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>