Updating AVR programmer support.

1. Documentation cleanup.
2. Avoid clock pulse when addressing.
Fixed time period (RTC/Watchdog::since) calculate.

1. Updated compiler test code for unsigned number comparison on wrap-around.
2. Updated RTC/Watchdog::since() to calculate the correct difference when the
timer wraps around.
3. Verifying truncated counter (e.g. 32-bit to 16-bit) and calculation.