depmod: use cleanup attribute to simplify free on exit

depmod: use cleanup attribute to simplify free on exit

Reusing the root variable was a bad idea. Doing so we could call free()
on a variable that was not allocated. For example: "depmod -b / -h".
Since we would jump to cmdline_failed, root would not be duplicated.

Instead of fighting the order in the options, just used the cleanup
attribute and remove the calls to free() on "config_paths" and "root".