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* SDMMC frequency selection based on board type

On Olimex ESP32 EVB I/O operations with SD card can cause error when LAN is used in same time.
Problem is disappearing if SD MMC frequency lower down from SDMMC_FREQ_HIGHSPEED to SDMMC_FREQ_DEFAULT.

No problem if WiFi used instead LAN.

* Code rewritten according to https://github.com/espressif/arduino-esp32/pull/5688#pullrequestreview-759359645