lbmk: don't handle TMPDIR directly

lbmk creates TMPDIR as /tmp/xbmk_*, but it's theoretically
possible that something could re-export it by mistake.

this change retains the same initialisation, but further
use is now via a new variable "xbmktmp", that stores the
value of TMPDIR upon lbmk's initialisation of it.

this reduces the chance of such a bug in the future, as
described above, so it is a preemptive/preventative fix.

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

include/vendor.sh

@@ -48,11 +48,11 @@ vendor_download()
 
 readkconfig()
 {
-	x_ rm -f "$TMPDIR/cbcfg"
+	x_ rm -f "$xbmktmp/cbcfg"
 	cbcfg="`check_defconfig "$boarddir"`" || for cbc in $cv; do
-		grep "$cbc" "$cbcfg" 1>>"$TMPDIR/cbcfg" 2>/dev/null || :
+		grep "$cbc" "$cbcfg" 1>>"$xbmktmp/cbcfg" 2>/dev/null || :
 	done
-	eval "`setcfg "$TMPDIR/cbcfg" 1`"
+	eval "`setcfg "$xbmktmp/cbcfg" 1`"
 
 	for c in $cvchk; do
 		eval "[ \"\${$c}\" = \"/dev/null\" ] && continue"