stand/lua: Correct interpretation of autoboot_delay

autoboot_delay=NO is documented to wait for input and *not* autoboot, which
is the exact opposite of the current behavior.

Additionally, autoboot_delay=-1 is documented to disallow the user from
interrupting the boot (i.e. autoboot immediately), which was not previously
honored.

This also fixes the case insensitive comparison to be truly case
insensitive. This is kind of nit-picky, but the previous version would only
accept "no" and "NO".