karo: tx6: enable silent console

Silence console output to avoid irritation of equipment connected to
the console UART.
Set the variable 'silent' in the compiled-in environment, so that
actually _all_ console output is silenced. Otherwise 'silent' would
only be activated after reading the env from mass storage.

Note: to reactivate console output for debugging purposes from the
command prompt, the env variable 'silent' has to change state from
defined to undefined. So, if the variable is not set in the saved
environment, it has to be set (to an arbitrary value) and removed
afterwards.