implement uptimeToString()

[mister-grumbler]

Implementation of
@brief Constructs string that represents current uptime using given format.
@param strBuff
A pointer to a string buffer where the result should be placed.
@param format
Day - D | d, Hour - H | h, Minute - M | m, Second - S | s.
In place of capital letter the actual value will be shown even if this value is zero. The small letter will be replaced by actual value only if this value is non-zero.
Due to the limited display size, only the "." character is allowed as a separator.
Example: "dd.hH.MM" for 00 days, 10 hours and 02 minutes will produce ".10.02" result.