Reported in https://github.com/espressif/esp-idf/issues/3299
If a config item contains a reserved filename in Windows like _CON_ then an invalid
directory is created.
We don't rely on this feature all, so disable it.
Fix warnings where undefined vars are used.
Make Kconfig emit "FOO=" for unset bool options
To ensure make variables are always defined, even if empty.
When writing auto.conf, include symbols disabled by dependency to make sure all make variables are always defined.
Fixesespressif/esp-idf#137
Cherry-picked from https://github.com/espressif/esp-idf/pull/138
