Update isolinux.cfg

Was getting a screen asking which keymap to use.  Adding either or both of auto=true and console-keymaps-at/keymap="American English".  This could have been caused by my changes to preseed.cfg.  However, my research online shows that the keyboard settings in the forked version of preseed.cfg are relevant for Debian Squeeze, and will no longer work in Wheezy.  http://debian.2.n7.nabble.com/Preseeding-keyboard-configuration-issue-td2826528.html