reeBSD venus.ifong.org 12.1-RELEASE FreeBSD 12.1-RELEASE MYKERNEL  amd64
--
cd /usr/src/tools/tools/nanobsd/embedded/
sh ../nanobsd.sh -c ./qemu-i386.cfg
# default will clean all (under obj)
# then buildworld (take 5 hrs at vultr)
# then buildkernel

detail
--
less nanobsd.sh or nanobsd.sh -h
nanobsd.sh 	-w      #suppress buildworld (comment: no buildworld, but do compile kernel)
        	-b      #suppress builds (both kernel and world) (comment: no kernel, no world)

default.sh can change kernel config
 cat default.sh | grep GENERIC
 @venus-> cat /usr/src/tools/tools/nanobsd/defaults.sh | grep NANO_KERNEL=