[CMD][SECO_CONFIG] Fix accidental modification of environment variables

The strtok was directly called on the pointer returned from the uboot env function, which is directly giving access to the cached data. As strtok modifies the string it gets by inserting a \0 at the split, followup reads of the same variable get a different result (only the first element before the split. To fix this, the string is now duplicated before spliting.