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.
Okay. How about `<SYSLINUX_DEFAULT_MODE>`?

It follows the theme of the other variable, where the string to replace is also the name of the variable.