Was able to set up a decent USB-Stick to boot a mini-Debian. Had a little struggle finding correct boot environment vars. Found some in this forum. I'm able to boot from flash again with a matching Achlinux environment. Thanks @Joe and @bodiby connaisseur - uBoot
I've done something wrong with NAND flashing latest Arch Linux U-Boot in stock env on a NSA325v2. Now the box isn't self-booting / self-hosting anymore. But with kwboot and a decent U-Boot kwb file I'm able to boot the box and have access to a "NSA325>"-prompt. Now how to write the U-Boot inside kwboot to correct addresses? Plus I'd could use some adviseby connaisseur - uBoot