kconfig: fix savedefconfig with choice marked optional

savedefconfig failed to save the correct minimal config
when it encountered a choice marked optional.

Consider following minimal configuration:
$cat Kconfig
choice
	prompt "choice"
	optional

config A
	bool "a"

config B
	bool "b"

endchoice

$cat .config | grep -v ^#
CONFIG_A=y

$conf --savedefconfig=defconfig Kconfig

would before this fix result in an empty file, because
kconfig would assume that CONFIG_A=y is a default value.
But because the choice is optional the default is that
both A and B are =n.

Fix so we handle optional choices correct.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/kconfig/confdata.c

@@ -497,15 +497,17 @@ int conf_write_defconfig(const char *filename)
 			/*
 			 * If symbol is a choice value and equals to the
 			 * default for a choice - skip.
-			 * But only if value is bool and equal to "y" .
+			 * But only if value is bool and equal to "y" and
+			 * choice is not "optional".
+			 * (If choice is "optional" then all values can be "n")
 			 */
 			if (sym_is_choice_value(sym)) {
 				struct symbol *cs;
 				struct symbol *ds;
 
 				cs = prop_get_symbol(sym_get_choice_prop(sym));
 				ds = sym_choice_default(cs);
-				if (sym == ds) {
+				if (!sym_is_optional(cs) && sym == ds) {
 					if ((sym->type == S_BOOLEAN) &&
 					    sym_get_tristate_value(sym) == yes)
 						goto next_menu;