Display a warning message if a "lost+found" directory exists

"lost+found" folder is automatically created by `mk2fs`, and it's been
known to break build determinism if previously existing.

Test: run apexer with and without a "lost+found" directory and check for
warning message
Bug: 227794452

Change-Id: Ia01a42a4089050162502a19109e7770711cff93e
1 file changed