Follow the 6 steps to add support for a new target.
Step 1: Locate input and output pins.
Step 2: Add pin constraint file. This covers clock input, LED and UART
output.
Step 3: Create a clock generator. The chip has no internal clock
generator so the external 12 MHz clock is used.
Step 4: Add top level servant_ice40_cm36 which connects the one-wire
output of servant to the LED pin.
Step 5: Add fileset including the new top level and pin constraints.
Step 6: Add target icesugar-nano.
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