init.sh: Explicitly create the xbmktmp directory

mktemp would normally do it, but we must not rely on that

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

include/init.sh

@@ -197,7 +197,7 @@ xbmk_create_tmpdir()
 
 	# /tmp might be a tmpfs, so for large files we use ./tmp,
 	# not to be confused with xbmktmp (xbmktmp points to /tmp)
-	x_ mkdir -p tmp
+	x_ mkdir -p "$xbmktmp" tmp
 }
 
 xbmk_lock()