usb: Copy image file to /tmp/images via USB

Traverse the USB mount directory, and copy the first file with
a `.tar` extension under the directory to the `/tmp/images`
directory.

Tested: Manually call the fs::copy_file app locally, test passed.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I8e03b9c3305f5e9d9a1535dd176ecd40dbea8903
4 files changed