kconfig: list unknown symbols in the old .config

[Sergey Senozhatsky]

Hi,

We recently were hit (unnecessarily hard) when after kernel uprev we
figured that something wasn't working. The root cause was a rename of
the CONFIG_FOO option between kernel releases, which make oldconfig
doesn't warn/notify about.

Would it be possible to add either a new --listunknown mode to conf or
to somehow make it conf_warning("unknown symbol: %s\n", line) when it
reads a line from oldconf that it cannot sym_find()?

That would save a ton of time.