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This enables us to move forward to a cleaner implementation,
where the "core" fel.c code will become independent of direct
libusb usage. After moving USB code to a separate module,
in the end the libusb handle could become an 'opaque' field of
feldev_handle.

The "device" handle might also be extended later, to provide
(FEL) version data and SoC-specific information (chip ID, SRAM
info, human-readable name).
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>