When CHROOTBUILD_SKIP is set, evaluate the existence of /bin/sh

within the CHROOTDIR.  If it does not exist, unset CHROOTBUILD_SKIP
to prevent build failures.

Requested by:	swills
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

release/release.sh

@@ -203,6 +203,11 @@ env_check() {
 		exit 1
 	fi
 
+	# Unset CHROOTBUILD_SKIP if the chroot(8) does not appear to exist.
+	if [ ! -z "${CHROOTBUILD_SKIP}" -a ! -e ${CHROOTDIR}/bin/sh ]; then
+		CHROOTBUILD_SKIP=
+	fi
+
 	CHROOT_MAKEENV="${CHROOT_MAKEENV} \
 		MAKEOBJDIRPREFIX=${CHROOTDIR}/tmp/obj"
 	CHROOT_WMAKEFLAGS="${MAKE_FLAGS} ${WORLD_FLAGS} ${CONF_FILES}"