vfs: symlink support in file picker, test cleanup, boot mount fix

- File picker (app.filepicker.pas): add otSYMLINK to directory case in
  fp_collect_cb so symlinks like /sys appear as navigable directories
- VFS: add RemoveVirtualTree() to recursively free vdir/symlink trees
- VFS UnitTest: clean up /utest_vfs tree after all tests
- STORTEST: clean up /st_test tree after tests, fix listing leak in
  cmd_stortest (GetDirectoryListingFrom result was never freed)
- Boot mount: auto-mount boot drive at /boot via mountVolume() instead
  of symlink; remove /boot and /cfg from init() vdirs
- mount.asr: remove boot line (now only 'mnt {device}/sys /sys')

iso/mount.asr

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