This article explains how to load images to the target over UART in U-Boot. Keep in mind that loading via a serial port will take quite a long time (minutes per file!) due to the low speeds (limited to 115.2 Kps). That said, for embedded designs that do not provide an Ethernet port it may be the only reasonable development and software manufacturing option using U-Boot and uClinux.
Using the NXP i.MX RT1170 RT EVK board, you connect to the target serial console available on the J23 corresponding connector on the baseboard. Assuming you connect to a Linux PC host, on the Linux host the i.MX RT1170 RT serial console will be visible as a /dev/ttyACMx device. The U-Boot and Linux software configures the console for 115.2 Kps operation.
...