kconfig: introduce choice_set_value() helper

Currently, sym_set_tristate_value() is used to set 'y' to a choice
member, which is confusing because it not only sets 'y' to the given
symbol but also tweaks flags of other symbols as a side effect.

Add a dedicated function for setting the value of the given choice.

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