Re-burning the bootloader

[NapoleonBorn2Party]

Sometime ago I followed a tutorial to use the MKS SBASE v1.3 with RRF using a Raspberry Pi as a SBC. Now I want to go back to using Marlin firmware on the board, however I can no longer flash a new firmware using the conventional method of plugging a SD card containing the firmware.bin and powering the board. I suspect the bootloader is no longer functioning (due to the modifications done to use RRF). How can I reflash the bootloader? I can't find an ISP interface on the board, nor the 6 pin uart interface. All there seems to be is a JTAG connection, but I'm unsure if I can use that to burn a bootloader...

[DivingDuck]

within this rar file you can find the bootloader.
https://github.com/makerbase-mks/MKS-SBASE/tree/master/English/Source

Or go to the original source:
https://github.com/Smoothieware/Smoothieware/tree/edge/bootloader