[K12OSN] How to make a Mac default to network boot

[Petre Scheie]

How do I make a Mac default to network boot, so I don't have to hold 
down the N key during boot?

(BTW, I just got this Mac, a castoff from a neighbor.  I put a new disk 
in a spare server, did a network install of K12LTSP 4.2.1, and 30 
minutes after I got the Mac it's booting as a Linux terminal!  That is 
so cool!  Congrats & high-fives to Eric, Jim K. Chuck, Trask, and 
everyone else for making the Mac clients possible.  I can't wait to show 
the school.)

Petre