lbmk: use mkhtemp in libreboot's build system

i added a fake -t option, which doesn't actually
read optarg, so that -t usage can just override
the normal template. mkhtemp isn't ready for
distros yet, but it's ready for lbmk.

i hacked the makefile to also copy the binary to
mktemp, and i set PATH in lbmk so that this binary
is used insttead of the one on your system.

that way, upstream projects use it.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/tree.sh

@@ -444,7 +444,7 @@ project_up_to_date()
 		    "$@"
 	fi
 
-	build_sbase
+	bootstrap_utils
 	fx_ "x_ util/sbase/sha512sum" find "$@" -type f -not -path \
 	    "*/.git*/*" | awk '{print $1}' > "$xbtmp/tmp.hash" || \
 	    err "!h $project $hashdir" \