Be aware that there are two bootloaders (u-boot, and u-boot bist). The smaller u-boot is designed to load and run completely inside the SoC (RAM in the same chip as the CPU). After the small u-boot initializes SDRAM, it loads into SDRAM the bigger u-boot bist (built-in-self test), which contains fastboot protocol and other goodies.
Things are not always as simple as you might expect.
