kconfig: add warn-unknown-symbols sanity check

kconfig: add warn-unknown-symbols sanity check

[ Upstream commit 7cd343008b967423b06af8f6d3236749c67d12e8 ]

Introduce KCONFIG_WARN_UNKNOWN_SYMBOLS environment variable,
which makes Kconfig warn about unknown config symbols.

This is especially useful for continuous kernel uprevs when
some symbols can be either removed or renamed between kernel
releases (which can go unnoticed otherwise).

By default KCONFIG_WARN_UNKNOWN_SYMBOLS generates warnings,
which are non-terminal. There is an additional environment
variable KCONFIG_WERROR that overrides this behaviour and
turns warnings into errors.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: a409fc1463d6 ("kconfig: fix memory leak in sym_warn_unmet_dep()")
Signed-off-by: Sasha Levin <sashal@kernel.org>