kconfiglib.py _save_old() may rename /dev/null -- replacing /dev/null with a file

[domo141]

Describe the bug

the kconfiglib.py _save_old("/dev/null") may actually, unexpectly, succeed.

afterwards /dev/null was opened for writing as a (new) file

To Reproduce
Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -DBOARD=board_xyz
3. ls -l /dev/null
4. cat /dev/null

Expected behavior

/dev/null is still device node

Impact

/dev/null is a file, with kconfig content

What impact does this issue have on your progress

host code that opened /dev/null as config file got "garbage"

Environment (please complete the following information):

• a docker container (not as root, but probably enough capabilites to rename /dev/null)

zephyr v1.14 -- but after investigating main in 2021-01 it still can happen

** how to fix **

if not os.path.isfile(path) return to skip rename if path does not point to a file
(either directly, or via symlink)

[nashif]

@domo141 Can you please submit a PR with the proposed fix?

[domo141]

Created (as usual, change was simple, comments and commit message hard)...

#31438: based on commit msgs ulfalizer may (or may not) follow up soon...

[nashif]

lowering priority as this happens only if /dev/null is writable or as root, which is not recommended.