Move RYMCU BigSmart under manufacturer directory

Create main/boards/rymcu/bigsmart so future RYMCU boards can live under the same manufacturer directory. Update CMake to set MANUFACTURER to rymcu while preserving BOARD_NAME as rymcu-bigsmart, and adjust config.json so release output remains rymcu-bigsmart.

main/boards/rymcu/bigsmart/README.md

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+# RYMCU BigSmart
+
+该目录为 `RYMCU BigSmart` 开发板适配，并按以下硬件资源完成映射：
+
+- 主控：ESP32-S3-WROOM-1-N16R8
+- 显示：ST7789（320x240，SPI）
+- 触摸：GT911（I2C）
+- 音频：ES8311 + ES7210（I2S + I2C）
+- IO扩展：PCA9557（I2C 地址 `0x19`）
+- 摄像头：GC0308（DVP）
+
+参考硬件文档：
+
+- https://github.com/rymcu/BigSmart-Open/blob/main/docs/rymcu-bigsmart-hardware.md
+
+## 编译
+
+```bash
+idf.py set-target esp32s3
+idf.py menuconfig
+```
+
+在菜单中选择：
+
+`Xiaozhi Assistant -> Board Type -> RYMCU BigSmart`
+
+然后执行：
+
+```bash
+idf.py build
+```