[U-Boot] Store ENTER ('\r') character in bootstopkey variable

[Giuseppe Modugno]

I'd like to use the ENTER key to stop autoboot through the variables 
"bootdelaykey" and "bootstopkey".  How could I do this?

The problem is to type escape characters ('\r') on the prompt line.  I 
tried with several possibilities, but nothing worked.

Any suggestions?