Our Missile Defense Agency is currently working on a boost-phase-intercept system using unmanned aerial vehicles equipped with high-energy lasers — but that plan is years away, since the lasers it would need to succeed don’t yet exist. That said, a UAV stationed at an altitude of 50,000–55,000 feet, 350 miles outside North Korean air space, carrying infra-red sensors and a conventional high-speed interceptor missile of 500 lbs. would still have the range, firepower, and time needed to bring down even a large North Korean ICBM in its boost phase.
Yes, a boost-phase kill should be the first layer of defense (well, air/missile offensive strikes and cyber-attacks would technically be the first line of defense) going all the way back to point defenses as the last line.
I described what I'd read would be the elements of the layered defense quite some time ago. That was in relation to Iran, but the layers apply to North Korea, too.
