Modify nanobsd/defaults.sh to reduce buildsize, buildtime and
increase imgsize to fit.

Not building debug symbols and tests reduces _.w/ from 1465 to 503 MB.
Increasing the mediasize to 6000000 let the image be built.
Otherwise I get:
...
00:33:58 build code slice 00:33:58 # log: /usr/obj/nanobsd.full/_.cs

/usr/obj/nanobsd.full/_.mnt: write failed, filesystem is full

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D49885