kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG

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