kconfig: do not write choice values when their dependency becomes n

"# CONFIG_... is not set" for choice values are wrongly written into
the .config file if they are once visible, then become invisible later.

  Test case
  ---------

---------------------------(Kconfig)----------------------------
config A
	bool "A"

choice
	prompt "Choice ?"
	depends on A

config CHOICE_B
	bool "Choice B"

config CHOICE_C
	bool "Choice C"

endchoice
----------------------------------------------------------------

---------------------------(.config)----------------------------
CONFIG_A=y
----------------------------------------------------------------

With the Kconfig and .config above,

  $ make config
  scripts/kconfig/conf  --oldaskconfig Kconfig
  *
  * Linux Kernel Configuration
  *
  A (A) [Y/n] n
  #
  # configuration written to .config
  #
  $ cat .config
  #
  # Automatically generated file; DO NOT EDIT.
  # Linux Kernel Configuration
  #
  # CONFIG_A is not set
  # CONFIG_CHOICE_B is not set
  # CONFIG_CHOICE_C is not set

Here,

  # CONFIG_CHOICE_B is not set
  # CONFIG_CHOICE_C is not set

should not be written into the .config file because their dependency
"depends on A" is unmet.

Currently, there is no code that clears SYMBOL_WRITE of choice values.

Clear SYMBOL_WRITE for all symbols in sym_calc_value(), then set it
again after calculating visibility.  To simplify the logic, set the
flag if they have non-n visibility, regardless of types, and regardless
of whether they are choice values or not.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>

scripts/kconfig/symbol.c

@@ -371,11 +371,13 @@ void sym_calc_value(struct symbol *sym)
 		sym->curr.tri = no;
 		return;
 	}
-	if (!sym_is_choice_value(sym))
-		sym->flags &= ~SYMBOL_WRITE;
+	sym->flags &= ~SYMBOL_WRITE;
 
 	sym_calc_visibility(sym);
 
+	if (sym->visible != no)
+		sym->flags |= SYMBOL_WRITE;
+
 	/* set default if recursively called */
 	sym->curr = newval;
 
@@ -390,7 +392,6 @@ void sym_calc_value(struct symbol *sym)
 				/* if the symbol is visible use the user value
 				 * if available, otherwise try the default value
 				 */
-				sym->flags |= SYMBOL_WRITE;
 				if (sym_has_value(sym)) {
 					newval.tri = EXPR_AND(sym->def[S_DEF_USER].tri,
 							      sym->visible);
@@ -433,12 +434,9 @@ void sym_calc_value(struct symbol *sym)
 	case S_STRING:
 	case S_HEX:
 	case S_INT:
-		if (sym->visible != no) {
-			sym->flags |= SYMBOL_WRITE;
-			if (sym_has_value(sym)) {
-				newval.val = sym->def[S_DEF_USER].val;
-				break;
-			}
+		if (sym->visible != no && sym_has_value(sym)) {
+			newval.val = sym->def[S_DEF_USER].val;
+			break;
 		}
 		prop = sym_get_default_prop(sym);
 		if (prop) {