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.

Also, as a side-efect, this patch fixes the following case:

    ---8<---
    choice
    config OPTION_A
        bool "Option A"
    config OPTION_B
        bool "Option B"
    config OPTION_C
        bool "Option C"
    endchoice
    ---8<---

which could previously generate such .config files:

    ---8<---                            ---8<---
    CONFIG_OPTION_A=y                   CONFIG_OPTION_A=y
    CONFIG_OPTION_B=y                   # CONFIG_OPTION_B is not set
    # CONFIG_OPTION_C is not set        CONFIG_OPTION_C=y
    ---8<---                            ---8<---

Ie., the first entry in a choice is always set, plus zero or one of
the other options may be set.

This patch ensures that only one option may be set for a choice.

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: Arnaud Lacombe <lacombar@gmail.com>
---
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