boards: riscv: esp32c3_luatos_core: make usb variant

Converts the board above to use variant option.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>

boards/luatos/esp32c3_luatos_core/Kconfig.esp32c3_luatos_core_usb

@@ -1,5 +0,0 @@
-# Copyright (c) 2023 YuLong Yao<feilongphone@gmail.com>
-# SPDX-License-Identifier: Apache-2.0
-
-config BOARD_ESP32C3_LUATOS_CORE_USB
-	select SOC_ESP32C3_MINI_N4

boards/luatos/esp32c3_luatos_core/board.yml

@@ -1,9 +1,7 @@
-boards:
-  - name: esp32c3_luatos_core
-    vendor: luatos
-    socs:
-      - name: esp32c3
-  - name: esp32c3_luatos_core_usb
+board:
+  name: esp32c3_luatos_core
   vendor: luatos
   socs:
   - name: esp32c3
+    variants:
+    - name: usb

boards/luatos/esp32c3_luatos_core/esp32c3_luatos_core.yaml

@@ -1,4 +1,4 @@
-identifier: esp32c3_luatos_core
+identifier: esp32c3_luatos_core/esp32c3
 name: ESP32C3 LuatOS Core
 type: mcu
 arch: riscv

boards/luatos/esp32c3_luatos_core/esp32c3_luatos_core_esp32c3_usb.yaml

@@ -1,4 +1,4 @@
-identifier: esp32c3_luatos_core_usb
+identifier: esp32c3_luatos_core/esp32c3/usb
 name: ESP32C3 LuatOS Core USB
 type: mcu
 arch: riscv