You are mixing many "boot" phases.
So check, change and save U-Boot variable "boot_targets" to change order for U-Boot phase in your U-Boot variables from chosen from Bootrom phase (eMMC in your case).
Default SPI flash is empty.
- Bootrom phase booting order is "eMMC, SPI Nor Flash, SDMMC card, USB OTG". See RK3328TRM page 14 and can be "changed" only by killing device (like installing jumper that force GND to eMMCCLK signal).
Bootrom phase search valid "ID magic" and load/run sequentially DDR.bin,TPL.bin, SPL.bin and U-Boot.bin binaries.
- U-Boot phase order is determined by "bootcmd" script that uses "boot_targets" variable with "mmc0 mmc1 usb0 pxe dhcp" (mmc# can be SDCARD or eMMC it depends on dts for u-boot). eMMC can still be killed by jumper.
- Kernel phase chooses kernel from extlinux.conf (now on root ext4 /boot/extlinux/extlinux.conf and NOT on efi partition /extlinux/extlinux.conf).
So check, change and save U-Boot variable "boot_targets" to change order for U-Boot phase in your U-Boot variables from chosen from Bootrom phase (eMMC in your case).
Default SPI flash is empty.
I left this community in Aug 2019 due to PINE64 refusal to produce/deliver ROCK64-1G version 3 after more than one year of changing statuses to "planning", "evaluating", "releasing", "availability", "estimated availability" and finally "no schedule" 
. ROCK64 is dead platform without any advantage. Buy Raspberry PI 4 !
. ROCK64 is dead platform without any advantage. Buy Raspberry PI 4 ! 
