vfs: replace asr.mnt with mount.asr declarative boot mounts

- Add mount.asr parser in auto_mount_volumes: reads MOUNT.ASR from boot
  volume root, parses 'mnt {device}<src> <target>' lines, expands {device}
  to the boot volume's /disk/volN path, creates symlinks for each entry
- Remove old asr.mnt persistent mount logic from auto_mount_volumes
- Remove persistent mount write ([p] flag) from MOUNT shell command
- Remove hardcoded /boot mount for boot volumes (now driven by mount.asr)
- Add iso/mount.asr with default /boot and /sys symlinks
- Update doc/vfs.md with mount.asr documentation

iso/mount.asr

@@ -0,0 +1,2 @@
+mnt {device}/boot /boot
+mnt {device}/sys /sys