fix(pxe): Note difference between tty0 and tty1

tty0 is normally used with the console= option (enable kernel output on
the VGA console) but coreos.autologin= should be tty1 or greater since
the login getty is on a virtual terminal, not the VGA console itself.
Merge pull request #197 from marineam/tty

fix(pxe): Note difference between tty0 and tty1