Dear Arno Steffen,
In message AANLkTik0YOONY62q_egSFsTn1nLcQ17yCi_E7bkrief0@mail.gmail.com you wrote:
I would like to have some rescue boot, if flashing of uboot-environment, kernel or filesystem fails.
Why should "flashing of uboot-environment" fail? If you configure for redundant environment, you should always at least have the previous copy of the settings).
As for the rest, kernel and file system, these are application data from U-Boot's point of view, and you can use any method to update these - alternate copies stored in flash and toggled with environment variables, automatic update from USB mass storage devices, automatic updates from TFTP server etc. - Have a look at the manual for details.
My idea is to check a certain port (eth) for some time and if I receive a defined string not to continue the ususal boot process but execute some into uboot compiled bootcmds.
Why inventing another (square?) wheel when there are already several other options available?
Best regards,
Wolfgang Denk