Hago una copia de la configuración del kernel genérico y creo un enlace a la ruta donde se ubican normalmente estos archivos:

cd /usr/src/sys/i386/conf
mkdir /root/kernels
cp GENERIC /root/kernels/BSDBOX
ln -s /root/kernels/BSDBOX

Edito mi copia de configuración del kernel con el objetivo de optimizarlo un poco, eliminando entradas que no vaya a necesitar. En mi caso el contenido final del archivo es el siguiente:

cpu             I686_CPU
ident           BSDBOX

options         SCHED_ULE               # ULE scheduler
options         PREEMPTION              # Enable kernel thread preemption
options         INET                    # InterNETworking
options         SCTP                    # Stream Control Transmission Protocol
options         FFS                     # Berkeley Fast Filesystem
options         SOFTUPDATES             # Enable FFS soft updates support
options         UFS_ACL                 # Support for access control lists
options         UFS_DIRHASH             # Improve performance on big directories
options         UFS_GJOURNAL            # Enable gjournal-based UFS journaling
options         GEOM_PART_GPT           # GUID Partition Tables.
options         GEOM_LABEL              # Provides labelization
options         COMPAT_43TTY            # BSD 4.3 TTY compat [KEEP THIS!]
options         STACK                   # stack(9) support
options         SYSVSHM                 # SYSV-style shared memory
options         SYSVMSG                 # SYSV-style message queues
options         SYSVSEM                 # SYSV-style semaphores
options         P1003_1B_SEMAPHORES     # POSIX-style semaphores
options         _KPOSIX_PRIORITY_SCHEDULING # POSIX P1003_1B real-time extension
options         KBD_INSTALL_CDEV        # install a CDEV entry in /dev
options         STOP_NMI                # Stop CPUS using NMI instead of IPI
options         AUDIT                   # Security event auditing

# To make an SMP kernel, the next two lines are needed
options         SMP                     # Symmetric MultiProcessor Kernel
device          apic                    # I/O APIC

# CPU frequency control
device          cpufreq

# Bus support.
device          pci

# ATA and ATAPI devices
device          ata
device          atadisk         # ATA disk drives
device          atapicd         # ATAPI CDROM drives
options         ATA_STATIC_ID   # Static device numbering

# SCSI peripherals
device          scbus
device          da
device          pass

# atkbdc0 controls both the keyboard and the PS/2 mouse
device          atkbdc          # AT keyboard controller
device          atkbd           # AT keyboard

device          vga             # VGA video card driver

# syscons is the default console driver, resembling an SCO console
device          sc
device          agp             # support several AGP chipsets

# Power management support (see NOTES for more options)
# Add suspend/resume support for the i8254.
device          pmtimer

# PCI Ethernet NICs that use the common MII bus controller code.
# NOTE: Be sure to keep the 'device miibus' line in order to use these NICs!
device          miibus          # MII bus support
device          rl              # RealTek 8129/8139

# Pseudo devices.
device          loop            # Network loopback
device          random          # Entropy device
device          ether           # Ethernet support
device          pty             # Pseudo-ttys (telnet etc)

# USB support
device          uhci            # UHCI PCI->USB interface
device          ohci            # OHCI PCI->USB interface
device          ehci            # EHCI PCI->USB interface (USB 2.0)
device          usb             # USB Bus (required)
device          ugen            # Generic
device          ukbd            # Keyboard
device          ulpt            # Printer
device          umass           # Disks/Mass storage - Requires scbus and da

Es importante, siempre que se actualizan los sources, recompilar todo el sistema, tanto el kernel como el resto de binarios.

En primer lugar elimino restos de compilaciones anteriores, aunque en este caso no sería necesario por tratarse de la primera vez:

cd /usr/obj
chflags -R noschg *
rm -rf *

Recompilamos e instalamos todo de nuevo:

cd /usr/src
make buildworld
make buildkernel KERNCONF=BSDBOX
make installkernel KERNCONF=BSDBOX
shutdown now
cd /usr/src
mergemaster -p
make installworld
mergemaster
reboot