You may wish to see this thread for how to boot from SATA: Armbian Forum
Also including some info below:
setenv bootcmd 'scsi scan; scsi dev 0; ext4load scsi 0:1 $kernel_addr $image_name;ext4load scsi 0:1 $fdt_addr $fdt_name;setenv bootargs $console root=/dev/sdb1 rw rootwait; booti $kernel_addr - $fdt_addr'
In the post I include my u-boot config for booting from SATA. If it will be the only drive attached and you do NOT have USB drives connected it should be changed to reflect /dev/sda1 in your case.
I also used Boot from removable storage – Ubuntu as a reference, and if you type help while in u-boot its pretty easy to get help on the needed commands.
Hope this helps!
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