Add --warn-undefined-variables to MAKEFLAGS

Add --warn-undefined-variables to MAKEFLAGS

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.

Fixes espressif/esp-idf#137

Cherry-picked from https://github.com/espressif/esp-idf/pull/138