kconfig: refactor choice value calculation

Handling choices has always been in a PITA in Kconfig.

For example, fixes and reverts were repeated for randconfig with
KCONFIG_ALLCONFIG:

- 422c809f03f0 ("kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG")
- 23a5dfdad22a ("Revert "kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG"")
- 8357b48549e1 ("kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG")
- 490f16171119 ("Revert "kconfig: fix randomising choice entries in presence of KCONFIG_ALLCONFIG"")

As these commits pointed out, randconfig does not randomize choices when
KCONFIG_ALLCONFIG is used. This issue still remains.

[Test Case]

choice
prompt "choose"

config A
bool "A"

config B
bool "B"

endchoice

$ echo > all.config
$ make KCONFIG_ALLCONFIG=1 randconfig

The output is always as follows:

CONFIG_A=y
# CONFIG_B is not set

Not only randconfig, but other all*config variants are also broken with
KCONFIG_ALLCONFIG.

With the same Kconfig,

$ echo '# CONFIG_A is not set' > all.config
$ make KCONFIG_ALLCONFIG=1 allyesconfig

You will get this:

CONFIG_A=y
# CONFIG_B is not set

This is incorrect because it does not respect all.config.

The correct output should be:

# CONFIG_A is not set
CONFIG_B=y

To handle user inputs more accurately, this commit refactors the code
based on the following principles:

- When a user value is given, Kconfig must set it immediately.
Do not defer it by setting SYMBOL_NEED_SET_CHOICE_VALUES.

- The SYMBOL_DEF_USER flag must not be cleared, unless a new config
file is loaded. Kconfig must not forget user inputs.

In addition, user values for choices must be managed with priority.
If user inputs conflict within a choice block, the newest value wins.
The values given by randconfig have lower priority than explicit user
inputs.

This commit implements it by using a linked list. Every time a choice
block gets a new input, it is moved to the top of the list.

Let me explain how it works.

Let's say, we have a choice block that consists of five symbols:
A, B, C, D, and E.

Initially, the linked list looks like this:

A(=?) --> B(=?) --> C(=?) --> D(=?) --> E(=?)

Suppose randconfig is executed with the following KCONFIG_ALLCONFIG:

CONFIG_C=y
# CONFIG_A is not set
CONFIG_D=y

First, CONFIG_C=y is read. C is set to 'y' and moved to the top.

C(=y) --> A(=?) --> B(=?) --> D(=?) --> E(=?)

Next, '# CONFIG_A is not set' is read. A is set to 'n' and moved to
the top.

A(=n) --> C(=y) --> B(=?) --> D(=?) --> E(=?)

Then, 'CONFIG_D=y' is read. D is set to 'y' and moved to the top.

D(=y) --> A(=n) --> C(=y) --> B(=?) --> E(=?)

Lastly, randconfig shuffles the order of the remaining symbols,
resulting in:

D(=y) --> A(=n) --> C(=y) --> B(=y) --> E(=y)
or
D(=y) --> A(=n) --> C(=y) --> E(=y) --> B(=y)

When calculating the output, the linked list is traversed and the first
visible symbol with 'y' is taken. In this case, it is D if visible.

If D is hidden by 'depends on', the next node, A, is examined. Since
it is already specified as 'n', it is skipped. Next, C is checked, and
selected if it is visible.

If C is also invisible, either B or E is chosen as a result of the
randomization.

If B and E are also invisible, the linked list is traversed in the
reverse order, and the least prioritized 'n' symbol is chosen. It is
A in this case.

Now, Kconfig remembers all user values. This is a big difference from
the previous implementation, where Kconfig would forget CONFIG_C=y when
CONFIG_D=y appeared in the same input file.

The new appaorch respects user-specified values as much as possible.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>