Principles of DC/DC converter w/ jumper-selectable outputs?
There's a number of ways you can go about it:


Use an "adjustable" buck regulator and switch the adjustment resistors for different values through jumpers.
Use multiple buck regulators to go from your input voltage to all the output voltages in one go (all running in parallel - only one of them active at a time)
Use multiple buck regulators in a "tree" formation - one which for example goes from input voltage to say 24v, then one which takes the 24v to (for example) 18v, then another which takes the 18v to say 12v - or whatever voltage combinations you want.


Option 1 is probably the cheapest as you only need 1 regulator - picking your feedback resistors can be tricky though if you want precise voltages.

Option 2 is probably the most efficient if you want multiple voltages to be available at the same time.

Option 3 is most commonly used for a combination of buck and linear regulators - buck it down to a lower voltage then linear it down lower still to give a good clean output.

Oh, and by the very nature of the linear regulators there is no way to use them that isn't wasteful.  They do have their place though as mentioned in option 3 (no switching ripple).