The RAK831 module does work correctly with the RAK stack, so going with @OlivierG comments, I would definitely suggest to first get your RAK stack compiled and executed.
Second, the CRC check failure is ok, as it happens when there is no sender to send a lora packet. However when testing the initial config, your spi_test and rx_test shouldn't throw "concentrator failure" during execution. So i would request you to build the RAK stack first and see if the above two commands work and show you a correct output
BTW, I would like to understand if there is any particular requirement for running the RAK831 via FTDI instead of full speed SPI? If its an industrial use case, I would recommend getting a industrial chipset variants of the imx6 or from broadcom and run it on the board directly via SPI. Since the FTDI support is getting deprecated with the semtech gateway version code v3.2.0. Hence any changes to the sx1301 or future chipset revision will not have full support for the FTDI based development. Just a suggestion from my end.
Official semtech repo notes:
Added support for SX1301AP2 reference design (with FPGA and additional SX127x). When a FPGA is detected at startup, the HAL automatically adapts SPI communication requests (using SPI header or not).
Added util_spectral_scan diagnostic tool to scan the spectral band in background, where the LoRa gateway operates. (can only be used with SX1301AP2 or similar design). By default it uses the same SPI device as the one used by the HAL, but it can be changed depending on the hardware architecture on which it is used by updating the SPI_DEV_PATH constant defined in file util_spectral_scan/src/loragw_fpga_spi.c. Note: when using same SPI device from 2 applications, we rely on the host SPI driver and OS to properly handle concurrent SPI requests. It has been tested on Raspberry Pi / Raspbian with spi_bcm2708 driver.* Removed SPI FTDI support due to lack of performances to properly handle heavy packet traffic. Only native SPI usage is recommended.
HAL: added a check that SX1301 firmwares have been properly loaded at startup.