* Created a more flexible imgtool::datetime structure for use within Imgtool
This is intended to replace most usage of time_t
* Changing the granularity of imgtool_clock from 1ms to 100ns, as per Vas' suggestion
* Created arbitrary_datetime in timeconv.h to facilitate interpretation of datetime info
I concluded that invoking std::mktime on manually assembled std::tm is bad, because it is indeterminate how the std::tm members may be "dominant". This required that I go further in imgtool, and update a number of drivers and eliminate the parameter of imgtool::datetime that takes std::tm.
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