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/rom.sh

@@ -463,7 +463,7 @@ cprom()
 	irom="$tmprom"
 
 	if [ $# -gt 0 ]; then
-		irom="$(mktemp || err "!mk irom, $(echo "$@")")" || \
+		irom="$("$mkhtemp" || err "!mk irom, $(echo "$@")")" || \
 		    err "can't copy rom" "cprom" "$@"
 
 		x_ cp "$tmprom" "$irom" && cpcmd="mv"