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/vendor.sh

@@ -633,7 +633,7 @@ prep()
 
 mksha512()
 {
-	build_sbase
+	bootstrap_utils
 
 	if [ "${1%/*}" != "$1" ]; then
 		x_ cd "${1%/*}"
@@ -746,7 +746,7 @@ vfile()
 # must be called from a subshell
 check_vendor_hashes()
 {
-	build_sbase
+	bootstrap_utils
 
 	x_ cd "$tmpromdir"