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/release.sh

@@ -35,7 +35,7 @@ release()
 		err "already exists: \"$reldest\"" "release" "$@"
 	fi
 
-	vdir="`mktemp -d || err "can't make vdir"`" || \
+	vdir="`"$mkhtemp" -d || err "can't make vdir"`" || \
 	    err "can't make tmp vdir" "release" "$@"
 	vdir="$vdir/$version"