kconfig: change sym_change_count to a boolean flag

sym_change_count has no good reason to be 'int' type.

sym_set_change_count() compares the old and new values after casting
both of them to (bool). I do not see any practical diffrence between
sym_set_change_count(1) and sym_add_change_count(1).

Use the boolean flag, conf_changed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>