Align Kconfig and Kbuild logic to Linux 4.20 release with minimal impact
on files outside of this scope.

Our previous Kconfig sync was done by commit 587e4a42

 ("kconfig /
kbuild: Re-sync with Linux 4.19").

As part of this re-sync, a few related changes from previous Linux
releases were found to have been missed and merged in, and are not in
the following list.

The imported Linux commits are:

[From prior to v4.19]
b1e0d8b70fa3 kbuild: Fix gcc -x syntax
a4353898980c kconfig: add CC_IS_GCC and GCC_VERSION
469cb7376c06 kconfig: add CC_IS_CLANG and CLANG_VERSION

[From v4.19 to v4.20]
487c7c7702ab kbuild: prefix Makefile.dtbinst path with $(srctree) unconditionally
0d91bf584fe5 kbuild: remove old check for CFLAGS use
25815cf5ffec kbuild: hide most of targets when running config or mixed targets
00d78ab2ba75 kbuild: remove dead code in cmd_files calculation in top Makefile
23066c3f4e21 Compiler Attributes: enable -Wstringop-truncation on W=1 (gcc >= 8)
37c8a5fafa3b kbuild: consolidate Devicetree dtb build rules
80463f1b7bf9 kbuild: add --include-dir flag only for out-of-tree build
77ec0c20c7e0 kbuild: remove VERSION and PATCHLEVEL from $(objtree)/Makefile
74bc0c09b2da kbuild: remove user ID check in scripts/mkmakefile
4fd61277f662 kbuild: do not pass $(objtree) to scripts/mkmakefile
80d0dda3a4e5 kbuild: simplify command line creation in scripts/mkmakefile
fb073a4b473e kbuild: add -Wno-pointer-sign flag unconditionally
9df3e7a7d7d6 kbuild: add -Wno-unused-but-set-variable flag unconditionally
69ea912fda74 kbuild: remove unneeded link_multi_deps
7d0ea2524202 kbuild: use 'else ifeq' for checksrc to improve readability
04c459d20448 kconfig: remove oldnoconfig target
0085b4191f3e kconfig: remove silentoldconfig target
3f80babd9ca4 kbuild: remove unused cc-fullversion variable
2cd3faf87d2d merge_config.sh: Allow to define config prefix
076f421da5d4 kbuild: replace cc-name test with CONFIG_CC_IS_CLANG
6bbe4385d035 kconfig: merge_config: avoid false positive matches from comment lines

[From post v4.20]
885480b08469 Makefile: Move -Wno-unused-but-set-variable out of GCC only block

There are a number of changes related to additional warnings as well as
being able to drop cc-name entirely that have been omitted for now as
additional work is required first.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>