ldr utils used for UART booting is very mature and requires very little pherephral hardware support, there should be something we missed by accident, document link says "bfin-uclinux-ldr -l u-boot.ldr /dev/ttyUSB0 &&kermit -l /dev/ttyUSB0 -b 57600 -C connect" instead of " bfin-uclinux-ldr -l u-boot.ldr /dev/ttyUSB0 && kermit -C connect". Try to split this command into two parts if it still does not work。
1）bfin-uclinux-ldr -l u-boot.ldr /dev/ttyUSB0
2） once the loading is done, connect it to a terminal program that you are familar with, make sure to set it to 57600-8-n-1