Use existing mounts in sysc when available

This would be the case when bubblewrap is used for sandboxing the
bootstrap.

sysc/run.sh

@@ -18,11 +18,11 @@ create_fhs() {
     for d in bin lib sbin; do
         ln -s "usr/${d}" "/${d}"
     done
-    mkdir -p /etc /proc /run /sys /tmp /var
-    mount -t proc proc /proc
-    mount -t sysfs sysfs /sys
+    mkdir -p /etc /run /var
+    test -d /proc || (mkdir /proc && mount -t proc proc /proc)
+    test -d /sys || (mkdir /sys && mount -t sysfs sysfs /sys)
     # Make /tmp a ramdisk (speeds up configure etc significantly)
-    mount -t tmpfs tmpfs /tmp
+    test -d /tmp || (mkdir /tmp && mount -t tmpfs tmpfs /tmp)
 }
 
 populate_device_nodes ""