Test for possible issues when both ESP and SAMD try to access SD card concurrently

[gabebuckmaster]

Both SAMD and ESP are configured as masters on the SPI bus to the SD card. If, through software mis-configuration, they both try to access the SD card at the same time, it may cause hardware issues. Test this and see if it actually causes a problem. If it does, a series resistor in the SPI lines from each microcontroller may solve the issue.