Allow Downgrade from variant to more generic board

[Josverl]

If I try to flash a bloard with a custom variant I get the following error :

16:30:21 | ℹ️ Flashing ESP32_GENERIC_C3-FAT v1.27.0 to 1 serial ports
16:30:21 | ℹ️ Target ports: COM14
16:30:21 | ❌ Board ESP32_GENERIC_C3-FAT not found in board database
16:30:21 | ❌ Board ESP32_GENERIC_C3-FAT not found
16:30:21 | ⚠️ No firmware files found for board  version v1.27.0
16:30:21 | ℹ️ Downloading v1.27.0 firmware for  on COM14.
16:30:22 | ℹ️ Found 0 potentially relevant firmwares
16:30:22 | ❌ No relevant firmwares could be found on https://micropython.org/download
16:30:22 | ℹ️ versions=['v1.27.0'] ports=[''] boards=['', '']
16:30:22 | ℹ️ Please check the website for the latest firmware files or try the preview version.
16:30:22 | ⚠️ No firmware files found for board  version v1.27.0
16:30:22 | ❌ MPFlashError: Failed to download v1.27.0 firmware for  on COM14.

Board info :

[
    {
        "serialport": "COM14",
        "family": "micropython",
        "description": "ESP32C3 module with ESP32C3",
        "version": "1.28.0-preview",
        "port": "esp32",
        "cpu": "ESP32C3",
        "arch": "rv32imc",
        "mpy": "v6.3",
        "build": "preview.250.g68c2d4e457.dirty",
        "location": "1-5.1.1",
        "toml": {},
        "vid": 6790,
        "pid": 29986,
        "board": "ESP32_GENERIC_C3",
        "board_id": "ESP32_GENERIC_C3-FAT",
        "variant": "FAT"
    }
]

That is because there is no variant "FAT" defined in the upstream repo, and no build published

in this case mpflash should ask if I want to install another - available - variant for that same board.
Use should be asked for conformation - or select from a list of options.