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>

scripts: mkmakefile: apply changes from Linux kernel ...

The version of mkmakefile which has been used in barebox until now dates
back to commit fd5f0cd from the Linux
kernel git history (Tue May 2 12:33:20 2006 +0200).
This patch effectively includes all changes from the aforementioned
commit to the latest stable version of the Linux kernel:

git log --pretty=oneline --abbrev-commit fd5f0cd..v4.10 scripts/mkmakefile
06ed5c2 kbuild: Make scripts executable
9319f45 kbuild: support simultaneous "make %config" and "make all"
0ff3577 kbuild: silence generated makefile message
3c955b4 fixes for using make 3.82
d230124 kbuild: teach mkmakfile to be silent
1d3b3bf kbuild: scripts/mkmakefile: dynamic determination of output directory
971edcf kbuild: re-enable Makefile generation in a new O=... directory
18c32da kbuild: fix building with O=.. options
0b35786 kbuild: call make once for all targets when O=.. is used

In particular, the list of changes includes a fix for the following
Make warning:

  Makefile:18: *** mixed implicit and normal rules: deprecated syntax

GNU Make 3.82 raises an error instead of a warning and aborts the
make process. GNU Make 3.82 is still used today by distributions
like Red Hat Enterprise Linux 7.3.

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add scripts/mkmakefile from Linux. This is necessary if you want to ...

build out-of-tree.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>