kconfig: remove 'optional' property support

The 'choice' statement is primarily used to exclusively select one
option, but the 'optional' property allows all entries to be disabled.

In the following example, both A and B can be disabled simultaneously:

choice
prompt "choose A, B, or nothing"
optional

config A
bool "A"

config B
bool "B"

endchoice

You can achieve the equivalent outcome by other means.

A common solution is to add another option to guard the choice block.
In the following example, you can set ENABLE_A_B_CHOICE=n to disable
the entire choice block:

choice
prompt "choose A or B"
depends on ENABLE_A_B_CHOICE

config A
bool "A"

config B
bool "B"

endchoice

Another approach is to insert one more entry:

choice
prompt "choose A, B, or disable both"

config A
bool "A"

config B
bool "B"

config DISABLE_A_AND_B
bool "choose this to disable both A and B"

endchoice

Some real examples are DEBUG_INFO_NONE, INITRAMFS_COMPRESSION_NONE,
LTO_NONE, etc.

The 'optional' property is even more unnecessary for a tristate choice.

Without the 'optional' property, you can disable A and B; you can set
'm' in the choice prompt, and disable A and B individually:

choice
prompt "choose one built-in or make them modular"

config A
tristate "A"

config B
tristate "B"

endchoice

In conclusion, the 'optional' property was unneeded.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

kconfig: tests: add basic choice tests

The calculation of 'choice' is a bit complicated part in Kconfig.

The behavior of 'y' choice is intuitive. If choice values are tristate,
the choice can be 'm' where each value can be enabled independently.
Also, if a choice is marked as 'optional', the whole choice can be
invisible.

Test basic functionality of choice.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>