kconfig: Show unsatisfied deps. when assignments don't take

Show which dependencies are unsatisfied when symbols don't get their
assigned value.

For example, assume that FOO below is assigned with CONFIG_FOO=y. Note
that BAR, BAZ, and STR = "hmm" are 'n'.

    config FOO
           bool "foo"
           depends on BAR && BAZ && QAZ && STR = "hmm"

    config BAR
           def_bool n

    config BAZ
           def_bool n

    config QAZ
           def_bool y

    config STR
           def_string "zmh"

This now prints this warning:

    warning: FOO (defined at /home/ulf/z/z/Kconfig:10) was assigned the
    value 'y' but got the value 'n'. Check these unsatisfied
    dependencies: BAR (=n), BAZ (=n), STR = "hmm" (=n). See ...

Fixes: #21888

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>