[pxe] Display the "Press F8" prompt rather than displaying menu with timeout

[pxe] Display the "Press F8" prompt rather than displaying menu with timeout

The PXE spec dictates the rather ugly feature that we have to present
a DHCP-specified prompt string to the user, then wait to see if they
press F8 before displaying the menu.

This seems to me to be a significant retrograde step from the current
situation of displaying the menu with the timeout counting down
against the default selected boot option, but apparently the lack of
the "Press F8" prompt causes some confusion.