[build] Allow safe concurrent builds of .iso, .liso and .sdsk targets

The geniso, genliso and gensdsk scripts contain hard-coded temporary
directory names, and so could potentially collide with each other when
run as part of a concurrent build (e.g. "make -j 4").

Fix by using mktemp to generate suitable temporary directory names.

src/util/geniso

@@ -22,8 +22,7 @@ then
 fi
 out=$1
 shift
-dir=bin/iso.dir
-mkdir -p $dir
+dir=`mktemp -d bin/iso.dir.XXXXXX`
 cfg=$dir/isolinux.cfg
 cp -p $isolinux_bin $dir
 cat > $cfg <<EOF