Only build USB-related modules if MK_USB != no

This change moves the thunderbolt module and other USB modules under a
MK_USB != no conditional to ensure that users not desiring USB support
can easily build systems without USB-specific drivers using this knob.

MFC after:	1 week
Reviewed By:	imp
Differential Revision: https://reviews.freebsd.org/D55576

sys/conf/kern.opts.mk

@@ -47,6 +47,7 @@ __DEFAULT_YES_OPTIONS = \
     SOURCELESS_UCODE \
     SPLIT_KERNEL_DEBUG \
     TESTS \
+    USB \
     USB_GADGET_EXAMPLES \
     ZFS
 
@@ -207,6 +208,10 @@ MK_KERNEL_SYMBOLS:=	no
 MK_DTRACE:=	no
 .endif
 
+.if ${MK_USB} == "no"
+MK_USB_GADGET_EXAMPLES:= no
+.endif
+
 # Some modules only compile successfully if option FDT is set, due to #ifdef FDT
 # wrapped around declarations.  Module makefiles can optionally compile such
 # things using .if !empty(OPT_FDT)

sys/conf/kmod.mk

@@ -103,10 +103,14 @@ LINUXKPI_GENSRCS+= \
 	pci_iov_if.h \
 	pcib_if.h \
 	vnode_if.h \
-	usb_if.h \
-	opt_usb.h \
 	opt_stack.h
 
+.if ${MK_USB} != "no"
+LINUXKPI_GENSRCS+= \
+	usb_if.h \
+	opt_usb.h
+.endif
+
 LINUXKPI_INCLUDES+= \
 	-I${SYSDIR}/compat/linuxkpi/common/include \
 	-I${SYSDIR}/compat/linuxkpi/dummy/include \

sys/modules/Makefile

@@ -356,7 +356,7 @@ SUBDIR=	\
 	${_rtw89} \
 	rtwn \
 	rtwn_pci \
-	rtwn_usb \
+	${_rtwn_usb} \
 	${_rtwnfw} \
 	${_s3} \
 	${_safe} \
@@ -411,7 +411,7 @@ SUBDIR=	\
 	${_ufshci} \
 	uinput \
 	unionfs \
-	usb \
+	${_usb} \
 	${_vesa} \
 	${_vf_i2c} \
 	virtio \
@@ -710,6 +710,14 @@ SUBDIR+=	ktest
 SUBDIR+=	tests
 .endif
 
+.if ${MK_USB} != "no" || defined(ALL_MODULES)
+_rtwn_usb=	rtwn_usb
+_usb=		usb
+. if ${MACHINE_CPUARCH} == "amd64"
+_thunderbolt=	thunderbolt
+. endif
+.endif
+
 .if ${MK_ZFS} != "no" || defined(ALL_MODULES)
 SUBDIR+=	zfs
 .endif
@@ -938,10 +946,6 @@ _bcm283x_pwm=	bcm283x_pwm
 _neta=		neta
 .endif
 
-.if ${MACHINE_CPUARCH} == "amd64"
-_thunderbolt=	thunderbolt
-.endif
-
 .if !(${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} < 110000)
 # LLVM 10 crashes when building if_malo_pci.c, fixed in LLVM11:
 # https://bugs.llvm.org/show_bug.cgi?id=44351