Is this possible without a lot work? It looks like only one type of Flash is supported at any one time, and U-Boot, uImage, and rootfs must all reside on the the Flash type chosen. The following webpage suggests a different location for rootfs is possible (under SPI Flash), but it seems that SD/MMC or USB is assumed (no examples are given).
http://processors.wiki.ti.com/index.php/Booting_Linux_kernel_using_U-Boot
This is problematic because these media can be removed. U-Boot can only be configured with CONFIG_USE_SPIFLASH or CONFIG_USE_NAND, so it doesn't seem that it can be used to download uImage and flash to to SPI, and then download the rootfs and flash it to NAND.