You can set up UBoot to load linux kernel and initrd from the usb key. At least that is as long as your UBoot was compiled with USB support.
If not, then you may have to use some secondary bootloader.
And I don't agree that using USB keys is any worse than say CF, I have been using them for years for all x86 installs. Just don't use no-name ones, buy brand name flash drive and it will last forever (ROS is made to run from memory and to not cause unnecessary writes... total writes after 10 years is still less then 1GB).
My RouterOS is already licensed/installed to the USB flash drive, that is why I wanted to use it.. Brand-name.. Kinda, but it is already done.. Otherwise I agree..
Definitely has USB support.. Built-in updates are done using USB flash drives..
Otherwise, cool Thank you.