core/pkg-kconfig: allow saving config to a non-existing custom config file

A very interesting use-case for a kconfig-based package is to create a
custom (def)config file based on one bundled with the package itself,
like described in PR-8156:

    make menuconfig
     -> enable kernel, use an in-tree defconfig, save and exit
    make linux-menuconfig
     -> enable/disable whatever option, save and exit
    make menuconfig
     -> change to use a custom defconfig file, set a path, save and exit
    make linux-update-config
     -> should save to the new custom defconfig file

However, that is currently not possible, because the dependency chain
when saving the configuration goes back up to the (newly-set!) custom
(def)config file, which does not exist.

So, we break the dependency chain so that saving the configuration does
not depend on that file. Instead, we use a terminal rule that checks
that the configuration has indeed been done, and fails if not.

Closes #8156.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>