Update U-Boot
To update U-Boot with U-Boot via TFTP the file u-boot.bin has to be copied to the TFTP directory.
user@ubuntu:/.../src/u-boot$ sudo cp u-boot.bin /tftpboot/
The following command has to be executed in the target in U-Boot:
MX35 U-Boot > run prg_uboot
After replacing the old U-Boot the board must be reset. Therefore simply execute the command reset.
MX35 U-Boot > reset