Add support for configrable syslinux menu default

The default syslinux menu entry can be selected by specifying a
parameter in machine.make of a platform - SYSLINUX_DEFAULT_MODE.

This parameter is documented in machine/kvm_x86_64/machine.make.
Here and below it would be better if syslinux.cfg contained a specific pattern we could search and replace.  That file has basically become a template.

So instead of s/default rescue/..... how about  s/<SYSLINUX_MODE>/ or some other string enclosed in angle brackets <>, similar to what does done for <CONSOLE_PORT> and the other CONSOLE parameters.

Other than that this patch looks fine.