Building Bootable FreeBSD Images
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Set up the build-environment ...

 1. setenv MAKEOBJDIRPREFIX /blah
 2. setenv TARGET i386
 3. setenv TARGET_ARCH i386

Build everything (userland, and kernel) ...

 4. make buildworld
 5. make buildkernel KERNCONF=GENERIC

Prepare a flash disk ...

 6. fdisk -I da0
 7. fdisk -B da0
 8. bsdlabel -w da0s1 auto
 9. bsdlabel -B da0s1
10. newfs /dev/da0s1a
11. mount /dev/da0s1a /mnt

Install userland and kernel ...

12. make installworld DESTDIR=/mnt
13. make installkernel DESTDIR=/mnt KERNCONF=GENERIC INSTALL_NODEBUG=t

Added the extra files needed for boot ...

14. make distrib-dirs DESTDIR=/mnt
15. make distribution DESTDIR=/mnt
16. echo /dev/da0s1a / ufs rw 1 1 > /mnt/etc/fstab
17. echo ifconfig_DEFAULT=DHCP > /mnt/etc/rc.conf
18. echo hostname=demo >> /mnt/etc/rc.conf

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This text has been copied from a blog post by Warner Losh, available at:
http://bsdimp.blogspot.com/2007/10/building-bootable-freebsdi386-images.html