RB_CONFIG.
Now, the code should do the right thing in the following cases, when
kernel is compiled with INTRO_USERCONFIG:
* when booted without userconfig_script and without RB_CONFIG, present
intro screen, and wait for user input.
* when booted with userconfig_script and without RB_CONFIG, DON'T present
intro screen unless explicitly asked in userconfig_script, basing on
assumption that if a user loads userconfig_script, (s)he already
decided what parameters to configure. Proceed with booting.
* when booted without userconfig_script, and with RB_CONFIG, enter
configuration utility and wait for user input.
* when booted with userconfig_script, and with RB_CONFIG, execute all
commands from userconfig_script, and DON'T leave the config utility,
but wait for user input.
And finally, regardless of the combination of the above parameters,
when intro screen is invoked either first or next times, and user
chooses to go back to CLI interface, unblock the quit command.
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