kconfig: deduplicate code in conf_read_simple()

[ Upstream commit d854b4b2 ]

Kconfig accepts both "# CONFIG_FOO is not set" and "CONFIG_FOO=n" as
a valid input, but conf_read_simple() duplicates similar code to handle
them. Factor out the common code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: a409fc14 ("kconfig: fix memory leak in sym_warn_unmet_dep()")
Signed-off-by: Sasha Levin <sashal@kernel.org>