Reading Spotify creds and WiFi creds from an SD card

[MrOneTwo]

Since I'm some ESP32 boards (Wemos Lolin D32 Pro for example) have a SD card port. It might be a good idea to use that port for reading WiFi and Spotify credentials, instead of setting those during build.

The main problem is that, on the Wemos Lolin D32 Pro the SD card port is connected to the SPI port. It's the same SPI port as the one by the RFID reader. That's not that big of a problem since... it's just a bus... it's possible handle both RFID reader and the SD card port with that one SPI.

Pins listed below are used for SPI.

#define PIN_NUM_MISO 19
#define PIN_NUM_MOSI 23
#define PIN_NUM_CLK  18
#define PIN_NUM_CS    5

The creds could be stored in files spotify.txt, wifi.txt. Nothing fancy. The sd_card_example_main.c file in the esp-idf directory shows how to implement reading from the card. Seems pretty simple.