kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG

Currently, randconfig does randomise choice entries, unless KCONFIG_ALLCONFIG
is specified.

For example, given those two files (Thomas' test-case):

    ---8<--- Config.test.in
    config OPTIONA
        bool "Option A"

    choice
        prompt "This is a choice"

    config CHOICE_OPTIONA
        bool "Choice Option A"

    config CHOICE_OPTIONB
        bool "Choice Option B"

    endchoice

    config OPTIONB
        bool "Option B"
    ---8<--- Config.test.in

    ---8<--- config.defaults
    CONFIG_OPTIONA=y
    ---8<--- config.defaults

And running:
    ./scripts/kconfig/conf --randconfig Config.test.in

does properly randomise the two choice symbols (and the two booleans).

However, running:
    KCONFIG_ALLCONFIG=config.defaults \
    ./scripts/kconfig/conf --randconfig Config.test.in

does *not* reandomise the two choice entries, and only CHOICE_OPTIONA
will ever be selected. (OPTIONA will always be set (expected), and
OPTIONB will be be properly randomised (expected).)

This patch defers setting that a choice has a value until a symbol for
that choice is indeed set, so that choices are properly randomised when
KCONFIG_ALLCONFIG is set, but not if a symbol for that choice is set.
Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>

---
Changes v3 -> v4
  - fix previous issue where some choices would not be set, which would
    cause silentoldconfig to ask for them and was then breaking this
    workflow (as reported by Arnd and Sedat):
        KCONFIG_ALLCONFIG=foo.defconfig make randconfig
        make silentoldconfig </dev/nullo
    which I have tested (3h28min!) with:
        touch defconfig
        for(( i=0; i<10000; i++ )); do
            KCONFIG_ALLCONFIG=$(pwd)/defconfig make randconfig >/dev/null 2>&1
            make silentoldconfig </dev/null >/dev/null 2>&1 || break
        done
    which did not break at all.
  - change done in v3 (below) is already fixed by a previous patch

Changes v2 -> v3
  - ensure only one symbol is set in a choice

Changes v1 -> v2:
  - further postpone setting that a choice has a value until
    one is indeed set
  - do not print symbols that are part of an invisible choice

scripts/kconfig/confdata.c

@@ -288,8 +288,6 @@ int conf_read_simple(const char *name, int def)
 	for_all_symbols(i, sym) {
 		sym->flags |= SYMBOL_CHANGED;
 		sym->flags &= ~(def_flags|SYMBOL_VALID);
-		if (sym_is_choice(sym))
-			sym->flags |= def_flags;
 		switch (sym->type) {
 		case S_INT:
 		case S_HEX:
@@ -379,13 +377,13 @@ int conf_read_simple(const char *name, int def)
 			case mod:
 				if (cs->def[def].tri == yes) {
 					conf_warning("%s creates inconsistent choice state", sym->name);
-					cs->flags &= ~def_flags;
 				}
 				break;
 			case yes:
 				if (cs->def[def].tri != no)
 					conf_warning("override: %s changes choice state", sym->name);
 				cs->def[def].val = sym;
+				cs->flags |= def_flags;
 				break;
 			}
 			cs->def[def].tri = EXPR_OR(cs->def[def].tri, sym->def[def].tri);
@@ -791,6 +789,8 @@ int conf_write(const char *name)
 			sym_calc_value(sym);
 			if (!(sym->flags & SYMBOL_WRITE))
 				goto next;
+			if (sym_is_choice_value(sym) && !menu_is_visible(menu->parent))
+				goto next;
 			sym->flags &= ~SYMBOL_WRITE;
 
 			conf_write_symbol(out, sym, &kconfig_printer_cb, NULL);