Enable configuring target variants (#7019)

This change enables picking the right board configuration from the
variants/ folder. Previously, we would always pick the default
configuration (e.g., "esp32" instead of "heltec_wifi_lora_32_V2").

Fix partition names in board.txt (#7120)

* typo edit: 8M flash (FAT to SPIFFS) + FAT to FATFS

* more precize fatfs size in 16M flash partition

* Revert size in 8M with spiffs partition