Add ESP32-C3 SuperMini DX-LR20 variant

[KrOik]

Summary

This adds a new board variant for the ESP32-C3 SuperMini paired with a DX-LR20 module, which uses an LLCC68-compatible
radio.

The variant defines the ESP32-C3 pin mapping, LLCC68 radio configuration, USB CDC boot settings, and build
environments for the common MeshCore roles. It also extends the existing LLCC68 support so this radio can use the same
rx gain CLI controls and RF switch handling path as the SX126x-style radios.

Changes

• Added variants/esp32c3_supermini_dxlr20
• Added build environments for:
  • repeater
  • room server
  • companion radio over USB
  • companion radio over BLE
• Enabled LLCC68 support for radio.rxgain
• Fixed LLCC68 RF switch pin fallback handling

Hardware Notes

The DX-LR20 module used here does not rely on DIO3 TCXO power control. RX/TX switching is handled through external RF
switch pins.

Tested pin mapping:

Signal	GPIO
SCK	4
MISO	5
MOSI	6
NSS	7
BUSY	9
RESET	10
RXEN	1
TXEN	2

Testing

Built successfully:

• pio run -e ESP32C3_SuperMini_DXLR20_repeater -j 1
• pio run -e ESP32C3_SuperMini_DXLR20_room_server -t size -j 1
• pio run -e ESP32C3_SuperMini_DXLR20_companion_radio_usb -t size -j 1
• pio run -e ESP32C3_SuperMini_DXLR20_companion_radio_ble -j 1

Hardware checked on an ESP32-C3 SuperMini with DX-LR20:

• repeater firmware booted without reset loops
• BLE companion firmware advertised successfully

Related work: I did not find an existing ESP32-C3 SuperMini + DX-LR20 variant PR. This builds on the existing LLCC68 support and adds a tested board-specific variant.
#114 Add ProMicroLLCC68 builds
#552 promicro: added sensor role, removed non-functional llcc68 variant envs
#499 Xiao ESP32 C3 updates
#1568 Ported SerialBLEInterface to NimBLE from Bluedroid to support C3 variants
#1687 Added PowerSaving for all ESP32-based repeaters
#2131 Add error-aware SX126x receive hook