atmega: add Emakefun RF-Nano V3.0 board

[matpala]

Adds the Emakefun RF-Nano V3.0, an Arduino Nano V3.0 compatible board (ATmega328P @ 16 MHz, CH340G over USB-C) with an onboard nRF24L01+ (or Si24R1) on the hardware SPI bus.

• New board file boards/rf_nano_v3.zig: Nano pin map plus LED, NRF_CE, NRF_CS, SCK, HODI and HIDO aliases. Radio pins are taken from the V3.0 schematic; the radio IRQ pin is not connected.
• New vendor group boards.emakefun.rf_nano_v3, derived from the ATmega328P.
• New rf-nano-v3_blinky example.

Tested on real hardware: rf-nano-v3_blinky blinks the onboard LED (with the delay from #1063 applied locally).

Questions:

1. Is a vendor group emakefun fine for this board?
2. Should the board file duplicate the Nano pin map (as Nano/Uno do today) or import it?
3. Nothing reads pin_map yet, so the aliases are documentation only. Are they useful, or would you prefer typed pins exposed from the board?