I’m not positive, but I don’t think fastboot looks for other devices…it was meant to be simple and fast. whereas u-boot can be set up for whatever order desired. If you have installed u-boot (mine is from R19.3), and go to the u-boot prompt on serial console, you can display environment with the “print” command (or you can type “help” to see commands).
To start with, “boot_targets” lists what is searched: boot_targets=mmc1 mmc0 usb0 pxe dhcp
…where mmc1 is mmcblk1p1 (SD), mmc0 is mmcblk0p1 (eMMC), usb is obvious, pxe is NFS client/server boot, and I’m guessing dhcp is similar to pxe.
The “boot_targets” variable is used in “bootcmd”, where a loop iterates through “boot_targets” in the order listed. If you wanted to try a different order, you could use the “setenv” command to alter the order of “boot_targets”; the change would not be permanent, but just for one boot, unless you “saveenv”.