I bought one of those cheap RTL-SDR units a few months ago. There's no real kernel code required for it - all of the rtl-sdr code just uses the generic USB userland API which is shared between many operating systems.
So, getting it going was pretty easy:
# pkg install rtl-sdr
Then, using it to test ADSB is pretty easy:
# rtl_adsb -V -S
.. this is verbose and listens to short packets.
Where I live (near San Jose Airport!) I receive a lot of ADSB transmissions. It's quite interesting.
Ok, so next - what about something more GUI like? Someone's already done it - https://github.com/antirez/dump1090 . There's already a package for it:
# pkg install dump1090
# dump1090 --net --aggressive
Then, point a webserver at http://localhost:8080/ and watch!