Here is another quick tip to help people who haven't dealt much with UEFI BIOS and the quirks of devices and booting. I my self went on a learning experience over the past day or so trying to figure out something that is essentially quite silly that I overlooked.
Any device you want to boot from MUST have a UEFI partition on the device related to the operating system you intend to boot. I made the mistake of thinking I could just 'dd' the eMMC over to an SDcard and boot that SDcard, I was wrong, there seems to be some magic there. Basically, unless the UEFI partition exists and things are setup correctly on the device (SDcard, USB stick, USB hard drive), the BIOS will not even show the device as an option to select to boot from.
This seems backwards logic for me, as one would expect that even if the device isn't 'currently bootable' that you would still have the option to choose the device, be it 'USB device' or something similar and at least tell it to expect to boot from that location. Instead, this is not how it works and without the device being present with that UEFI partition in tact, you can't select to boot from the device and it will not show up as an option.
To note: using 'dd' to write any of the provided .img files provided by AtomicPi to an SDcard or USB stick should write this UEFI partition for you. If now, there are some tools out there made to do this for you. Rufus ( https://rufus.ie/ ) or From Windows ( https://www.thomas-krenn.com/en/wiki/Creating_Windows_UEFI_Boot-Stick_in_Windows ). I am sure there are many other options if you Google for 'UEFI USB bootable'.
Hope this helps someone!