Revert "build: scripts/config - update to kconfig-v5.6"

This reverts commit dcf3e63a35.

The kconfig update requires further testing and refinement until it can
remain in tree. Main problems are:

 - Recursive deps are now fatal instead of a warning
 - Previously legal syntax now leads to hard failures
 - It fails all package builds since multiple days

The updated kconfig implementation needs to cope with the current status
quo in the various package feeds before we can reconsider it for master.

It is not desirable that single broken packages can hard-fail the entire
build pipeline.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

scripts/config/README

@@ -1,24 +1,2 @@
-These files were taken from the Linux Kernel Configuration System at commit
-089b7d890f972f6b649fedc9259f6b93a18fb970 (Feb 4, 2020) and modified for the
-OpenWrt Buildroot:
- - removed nconf, gconf, tests and kernel configuration targets
- - adjusted the Makefile to compile outside the kernel
- - always use default file when running make all{no,mod,yes}config
- - added a 'reset' command to reset config when the target changes
- - allow config reading from & writing to a different file
- - allow 'source' command to use globs to include multiple files
- - don't write auto.conf and other files under include/ directory
- - reverted a commit to allow use of '/' & '.' in unquoted config symbols.
-   There are too many of those in OpenWrt right now.
- - reverted a commit that was issuing a warning when there were more than
-   one help text.  This is used in a few packages to use different texts
-   for the menuconfig help, and the ipkg package description.
- - reverted an upstream change that avoids writing symbols that are not
-   visible to .config, which breaks OpenWrt busybox's '.config' generation
-   logic.
- - use pre-built *.lex.c *.tab.[ch] files by default, to avoid depending on
-   flex & bison.  Rebuild/remove these files only if running make with
-   BUILD_SHIPPED_FILES defined
-
-For a full list of changes, see the repository at:
-https://github.com/cotequeiroz/linux/commits/openwrt/scripts/kconfig
+These files were taken from the Linux 3.9 Kernel
+Configuration System and modified for the OpenWrt Buildroot.