USB is loadable as a module as well.

sys/modules/usb/Makefile

@@ -0,0 +1,56 @@
+#       $FreeBSD$ 
+
+MAINTAINER	 = n_hibma@freebsd.org
+S		 = ${.CURDIR}/../..
+
+.PATH:		 $S/dev/usb $S/pci
+KMOD		 = usb
+SRCS		 = bus_if.h device_if.h usb_if.h usb_if.c \
+		   vnode_if.c vnode_if.h \
+		   opt_usb.h \
+		   hid.c hid.h usbhid.h \
+		   uhub.c \
+		   usb.c usb.h \
+		   usb_mem.h \
+		   usb_port.h \
+		   usb_quirks.c usb_quirks.h \
+		   usb_subr.c \
+		   usbdevs.h usbdevs_data.h \
+		   usbdi.c usbdi.h usbdivar.h \
+		   usbdi_util.c usbdi_util.h
+
+SRCS		+= uhci_pci.c uhci.c uhcireg.h uhcivar.h
+SRCS		+= ohci_pci.c ohci.c ohcireg.h ohcivar.h
+SRCS		+= opt_bus.h pci_if.h
+
+NOMAN		 =
+CFLAGS		+= -g
+CLEANFILES	+= bus_if.h device_if.h pci_if.h usb_if.h usb_if.c \
+		   vnode_if.c vnode_if.h \
+		   uhci_pci.o uhci.o \
+		   ohci_pci.o ohci.o \
+		   opt_usb.h
+
+opt_usb.h:
+	echo > opt_usb.h
+#	echo "#define USB_DEBUG 1" >> opt_usb.h
+#	echo "#define UHCI_DEBUG 1" >> opt_usb.h
+#	echo "#define OHCI_DEBUG 1" >> opt_usb.h
+opt_bus.h:
+	echo > opt_bus.h
+
+bus_if.h:
+	perl $S/kern/makedevops.pl -h $S/kern/bus_if.m
+device_if.h:
+	perl $S/kern/makedevops.pl -h $S/kern/device_if.m
+usb_if.h:
+	perl $S/kern/makedevops.pl -h $S/dev/usb/usb_if.m
+usb_if.c:
+	perl $S/kern/makedevops.pl -c $S/dev/usb/usb_if.m
+pci_if.h:
+	perl $S/kern/makedevops.pl -h $S/pci/pci_if.m
+
+usb_devs.h:
+	perl $S/kern/makedevlist.pl -h $S/dev/usb/usb.devs
+
+.include <bsd.kmod.mk>