kconfig/conf: fix randconfig setting multiple symbols in a choice

kconfig/conf: fix randconfig setting multiple symbols in a choice

Currently, randconfig may set more than one symbol in a given choice.
Given this config file:
    config A
        bool "A"
    if A
    choice
        bool "B/C/D"
    config B
        bool "B"
    config C
        bool "C"
    config D
        bool "D"
    endchoice
    endif # A

Then randconfig generates such .config files (case where A is not set is not
shown below for brevity), and where only the right-most .config is valid:
  CONFIG_A=y                  CONFIG_A=y                  CONFIG_A=y
  CONFIG_B=y                  CONFIG_B=y                  CONFIG_B=y
  CONFIG_C=y                  # CONFIG_C is not set       # CONFIG_C is not set
  # CONFIG_D is not set       CONFIG_D=y                  # CONFIG_D is not set

That is, in a randomised choice, the first symbol is always selected,
and at most one other symbol may be selected.

This is due to symbol randomised in a choice not being properly flagged
as having a value.

Fix that by flagging those symbols adequately: have a user-defined value,
and be not valid (to force recalculation of the symbol).

Note: if the choice is not conditional, then the randomisation is properly
done.

Reported-by: Matthieu CASTET <matthieu.castet@parrot.com>
Signed-off-by: Matthieu CASTET <matthieu.castet@parrot.com>
[yann.morin.1998@free.fr: independently re-done the same patch as Matthieu,
                          as pointed out by Sedat]
Cc: Arnaud Lacombe <lacombar@gmail.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>