build: don't rm TMPDIR if it's /tmp

we override TMPDIR, setting it to /tmp/xbmk*C

if it's just set to tmp, that means we didn't set it properly,
which is a bug.

this patch protects against deletion of /tmp under such a
fault condition, if it were ever to occur in the future.

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

build

@@ -127,7 +127,7 @@ fail()
 tmp_cleanup()
 {
 	[ "$xbmk_parent" = "y" ] || return 0
-	rm -Rf "$TMPDIR" || return 1
+	[ "$TMPDIR" = "/tmp" ] || rm -Rf "$TMPDIR" || return 1
 	rm -f lock || return 1
 }