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For unknown option-style arguments (starting with '-'), exit after
printing an error message.

This avoids situations where sunxi-fel would not report incorrect
options (with no FEL device attached/detected) and fail with
"Allwinner USB FEL device not found" instead, which is undesirable.

TODO: Might have to eventually migrate this to some better argument
parsing, e.g. getopt(3) or something similar.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>