[U-Boot] Rescue /alternative boot

[Arno Steffen]

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