tools/usb: Add .gitignore file

Add .gitignore file for untracked files in tools/usb.
Signed-off-by: NPrarit Bhargava <prarit@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/usb/.gitignore

+ffs-test
+testusb