[U-Boot] Rescue /alternative boot

I would like to have some rescue boot, if flashing of uboot-environment, kernel or filesystem fails. 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. Looking into the code doesn't give me an idea howto do this. Does someone have tried something simular and would share his experience? Thanks Arno