kconfig / kbuild: Re-sync with Linux 4.20

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 587e4a429698 ("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>