util/kconfig: Uprev to Linux 5.13's kconfig

This was originally several commits that had to be squashed into one
because the intermediate states weren't able to build coreboot:

 - one to remove everything that wasn't our own code, leaving only
   regex.[ch], toada.c, description.md and Makefile.inc.
 - one to copy in Linux 5.13's scripts/kconfig and adapt Makefile.inc
   to make the original Makefile work again.
 - adapt abuild to use olddefconfig, simplifying matters.
 - apply patches in util/kconfig/patches.
 - Some more adaptations to the libpayload build system.

The patches are now in util/kconfig/patches/, reverse applying them
should lead to a util/kconfig/ tree that contains exactly the Linux
version + our own 5 files.

Change-Id: Ia0e8fe4e9022b278f34ab113a433ef4d45e5c355
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37152
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

util/kconfig/patches/0010-reenable-source-in-choice.patch

@@ -0,0 +1,16 @@
+Kconfig 5.8 (since commit 09d5873e4d1f70202314b5fe40160f9b14b9d2d0)
+blocks using the source statement within choice but that's a pattern we
+use intensively. Re-enable it.
+
+Index: kconfig/parser.y
+===================================================================
+--- kconfig.orig/parser.y
++++ kconfig/parser.y
+@@ -134,6 +134,7 @@ stmt_list_in_choice:
+ 	| stmt_list_in_choice comment_stmt
+ 	| stmt_list_in_choice config_stmt
+ 	| stmt_list_in_choice if_stmt_in_choice
++	| stmt_list_in_choice source_stmt
+ 	| stmt_list_in_choice error T_EOL	{ zconf_error("invalid statement"); }
+ ;
+