[RFC PATCH SET] build system: update kconfig to bring new features and fixes

[Xabier Oneca -- xOneca]

Hi,

I've been upgrading the Kconfig tools to a more modern version bought
from Linux sources. I've reached to a state where all seems to work,
but I would like to ask for feedback, and testers.

I just used the latest commit available, which at the time of starting
this journey was 5ffe04ccd69a. Actually there are more commits
available now; they can be applied in future patches.

I did not bring the testsuite for Kconfig, as I deem not interesting
for Busybox.

Also, some dependencies were updated. Mainly from scripts/ folder.
They conform the core of the build system, so almost all of the build
system is updated too. I tried to also update the main Makefile, but
it is heavily customized and could not get it to work, so I left it
for future patches.

The patch set changes can be summarized as follows:

- Patches 1-3 are little preparations. They don't seem to break the
build system.
- Patch 4 is to copy the relevant files from Linux repo to Busybox.
- Patch 5 gathers all the changes that need to be made to make the new
Kconfig again compatible with Busybox, and to recover the custom
functionality.

I've tested and seem to work the following make targets:
  config, nconfig, menuconfig, allnoconfig, defconfig,
freebsd_defconfig, all, doc, busybox, install, baseline, bloatcheck,
check, sizes, objsizes, bigdata, stksizes.

allyesconfig gives me errors and had to disable RPC, PAM, SELinux,
warnings-as-errors and fsanitize, for it to work both with old and new
build system.

Could not test cross-compiling, so it may be a possible failing point.

Let me know your impressions about this first version...

Cheers,

Xabier Oneca_,,_
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