tests: test_dfu.py: Add example udev rule for host_usb_dev_node

If one does not already have a rule to create a custom device node when
a given device enumerates it can be useful to have udev create a
bus path based node to the entry in /dev/bus/usb that was just
enumerated.  Given that DFU itself does not require a /dev entry it is a
good idea to provide a rule that will generate one.
Signed-off-by: NTom Rini <trini@konsulko.com>

test/py/tests/test_dfu.py

@@ -80,6 +80,13 @@ ACTION=="add", SUBSYSTEM=="block", SUBSYSTEMS=="usb", KERNELS=="3-13", MODE:="66
 (You may wish to change the group ID instead of setting the permissions wide
 open. All that matters is that the user ID running the test can access the
 device.)
+
+c) An optional udev rule to give you a persistent value to use in
+host_usb_dev_node. For example:
+
+IMPORT{builtin}="path_id"
+ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="", SYMLINK+="bus/usb/by-path/$env{ID_PATH}"
+ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*", SYMLINK+="bus/usb/by-path/$env{ID_PATH}-port$env{.ID_PORT}"
 """
 
 # The set of file sizes to test. These values trigger various edge-cases such