Yes, once you reach the point where you have u-boot up and running you can boot your OS from almost anything you want, or rather u-boot can reach, e.g. over the network or a USB attached HDD... that is why the main idea of having the small SPI Flash is to store u-boot on it so you can boot from different media that don't need to include the 'boot' firmware, almost like on PC's with traditional "BIOS"
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