Add iCESugar-nano board

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.

data/icesugar_nano.pcf

@@ -0,0 +1,9 @@
+# 12 MHz external clock
+set_io i_clk        D1
+
+# LED
+set_io o_led        B6
+
+# UART
+set_io i_uart_rx    A3
+set_io o_uart_tx    B3