kbuild: resync top Makefile, mkmakefile, tags.sh with Linux 5.7-rc4 ...

The prologue of the build system is not specific to Linux or Barebox.
Paritially resync with Linux 5.7-rc4

- In Linux, $(objtree) is always '.', and $(srctree) might be relative.
  I am keeping both absolute for now to avoid unexpected breakage.

- I did not resync single target for now. We need to touch
  Makefile.build a lot if we want to resync it.

- 'export CDPATH=' is Barebox-only code, which was added by
  commit 6cc8d0544658 ("Makefile: disable CDPATH"). Keep it.

- KBUILD_SRC no longer exists in the upstream Linux. I replaced it
  with building_out_of_srctree.

- scripts/mkmakefile does not contain anything specific to Linux or
  Barebox. Resync it with Linux 5.7-rc4

- scripts/tags was imported from Linux 3.6 with some cherry-picks on
  top of that. Resync it with Linux 5.7-rc4

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

tags: Process Kconfig files in a single pass ...

This is an adoption of the linux kernel commit

| commit a281b85
| Author: Michal Marek <mmarek@suse.com>
| Date:   Wed Oct 14 11:17:13 2015 +0200
|
|     tags: Process Kconfig files in a single pass
|
|     Signed-off-by: Michal Marek <mmarek@suse.com>

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

tags: Do not try to index defconfigs ...

This is an adoption of the linux kernel commit

| commit ab9ca61
| Author: Michal Marek <mmarek@suse.com>
| Date:   Thu Oct 15 11:14:02 2015 +0200
|
|     tags: Do not try to index defconfigs
|
|     The defconfig files are in predictable locations, so there is no need to
|     index them. Plus, the script was only looking for files named
|     'defconfig', which only works on a few architectures nowadays.
|
|     Signed-off-by: Michal Marek <mmarek@suse.com>

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

scripts/tags.sh: drop __TestClearPage*() helpers ...

This is an adoption of the linux kernel commit

| commit 685eaad
| Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
| Date:   Fri Jan 15 16:52:10 2016 -0800
|
|     page-flags: drop __TestClearPage*() helpers
|
|     Nobody uses them.
|
|     Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
|     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
|     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Makefile: move ctags and cscope support to a shell script ...

The scripts/tags.sh file copied from linux-3.6.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>