The original work was done by Dale Rahn. See https://www.openbsd.org/armish.html.
RedBoot was originally funded by RedHat. It is GPL'd. RedBoot source code can be found at https://sources.redhat.com/redboot/. However this does not necessarily include the exact parameters needed to build it for a Thecus.
There is also the RedBoot User Manual, which can be found in PDF on the Web.
I have been given permission by Thecus in Taiwan to redistribute this file to OpenBSD users that need it on the proviso that you follow the directions very carefully!
See Chapter 2 of the Thecus N2100 Manual.
Make sure both the Thecus and the computer controlling it (and the network hub between them) all are on uninterruptible power supplies.
Just browse to your device in a browser, press the Login button on the first screen,
and go to System->Firmware Upgrade. Browse to where you have this
file stored on disk. It should look like this:
By downloading this file, you assume all responsibility for whatever happens to your Thecus!!
OK, the file is here.
Also here is the permission to reproduce:
From: Y.T. Lee (email redacted, someplace in thecus.com) Subject: RE: Thecus N2100 1.9.x firmware? Date: 07/24/07 11:05 Dear Ian, We are sorry that the updated Redboot caused your inconvenience on OpenBSD porting to Thecus N2100. Because we change the support of ATAG on memory size auto detection, so the kernel parameters were generated by Redboot automatically to provide the backward compatibility to old Thecus N2100s. Please find the attachment for the un-official F/W patch to replace the Redboot image with old one. You can load the patch as a F/W upgrade and it will re-program the Redboot then request you to reboot after finished the upgrade process. Please remember: DO NOT turn off N2100's power supply during the F/W upgrade process, otherwise you will need to send it back to us for repair. It is also OK to provide this image to other developers if required, but please help us to ask them be careful during F/W upgrade to avoid our possible RMA effort. Regards, -- Y.T. Lee Thecus Technology Corp. 15F., No. 79, Sec. 1, Sintai 5th Rd., Sijhih City, Taipei County, Taiwan
In short, I think that means "use at own risk". You have been warned!!