[PATCH] Boot-time entering prompt by typing "123" sequence rather than any button

This patch adds the possibility to enter the prompt at boot time by
typing the sequence of buttons "123" rather than a single button:
http://www.freebsd.org/~attilio/Sandvine/STABLE_8/boot123/boot123.diff

This is useful in the cases where a serial console is likely going to
be used which can carry on spourious character, leding to the prompt
erroneously.
This option is wrappered into the BOOT_PROMPT_123 option, in order to
maintain the current POLA.
This patch has been contributed back by Sandvine Incorporated.
Please review.

Thanks,
Attilio


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