UBOOT flash to SPI

[R@nchito]

Hello,

I had my Rock64 working with the Stretch Mate image. Also the latest Android was booting fine.
Now  I tried flashing Uboot to the SPI which seemed to work, cause the white LED was blinking.
Unfortunately now nothing is booting anymore. I tried to use the Uboot erase SPI image but it did nothing.

Is there an alternative way to erase the SPI ?

Could it be that the Uboot image eis faulty ??  How to recover my Rock64 now ??

thanks for help

Stefan