FBSD: Use GENERIC kernel headers by default

Currently, if --with-bsd-kernel-build is not specified during
configure, we do not set KERNBUILDDIR during the libafs build at all.
This means that we do not use an opt_global.h during the build.

For the GENERIC kernel, in the past this has worked well enough to
produce a working kernel module, but with FreeBSD 12, the GENERIC
kernel turns on the VIMAGE option by default. If our kernel module is
built without VIMAGE defined, our kernel module cannot be loaded into
GENERIC, since VIMAGE changes the definitions of some symbols that we
use (for example, trying to load such a libafs fails with the message
"link_elf_obj: symbol in_ifaddrhead undefined").

To allow the build to work by default for GENERIC kernels, without
needing any additional configure flags, change the libafs Makefile to
use the headers for the GENERIC kernel if no kernel build dir is
given. To do this, we create a directory in our build tree and
generate the opt_*.h headers in there (using config(8)), and specify
that directory as the kernel build dir.

This approach only works for GENERIC kernels, of course, but that is
the most common scenario. Users that want to build for a custom kernel
configuration still need to specify the actual kernel build dir with
--with-bsd-kernel-build.

Change-Id: If716d3f8ded9596d8e5e98bf2e58888921fc5f7f
Reviewed-on: https://gerrit.openafs.org/14001
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/libafs/MakefileProto.FBSD.in

@@ -60,13 +60,33 @@ WARNS= 1
 # We have some sketchy code that redeclares prototypes
 CWARNFLAGS+= -Wno-redundant-decls
 
+GEN_KBLD_DIR=@abs_builddir@/kconf-GENERIC
+AFS_OS_CLEAN=$(GEN_KBLD_DIR)
+
+# Starting with FreeBSD 12, opt_global.h is effectively required to build, even
+# for the GENERIC kernel. Normally this would be provided in the kernel build
+# dir (--with-bsd-kernel-build). If that dir was not specified, we can still
+# get the needed header files if we assume that the user is building for the
+# GENERIC kernel. So if we don't have a kernel build dir, generate the header
+# files in GEN_KBLD_DIR by using config(8), and specify that as the
+# KERNBUILDDIR. This won't always work, of course, but it has a chance at
+# working for the most common setups (when the user isn't using some custom
+# kernel). The kernel headers, of course, we assume are in $(KSRC).
+$(GEN_KBLD_DIR):
+	-mkdir $(GEN_KBLD_DIR)
+	cd $(KSRC)/$(MACHINE)/conf && \
+		/usr/sbin/config -d $(GEN_KBLD_DIR) GENERIC
+
 # setup for bsd.kmod.mk infrastructure
 .PATH:	${TOP_SRCDIR}/afs/FBSD
 KMODDIR=/boot/modules
 KERN_DEBUGDIR=/usr/lib/debug
 KMOD=	libafs
 SYSDIR= ${KSRC}
-.if !empty(KBLD)
+
+.if empty(KBLD)
+KERNBUILDDIR=$(GEN_KBLD_DIR)
+.else
 KERNBUILDDIR=${KBLD}
 .endif
 
@@ -83,7 +103,7 @@ DESTDIRS = single_destdir
 # generating headers and objects in this directory, which we want in $(KOBJ).
 # We also need the install target from bsd.kmod.mk in $(KOBJ), but the one
 # from Makefile.common in this directory.  A prime candidate for cleanup.
-setup:
+setup: $(KERNBUILDDIR)
 	-mkdir $(KOBJ)
 	-$(RM) $(KOBJ)/Makefile $(KOBJ)/Makefile.common
 	$(CP) Makefile $(KOBJ)/Makefile