docs: kbuild: convert docs to ReST and rename to *.rst

The kbuild documentation clearly shows that the documents
there are written at different times: some use markdown,
some use their own peculiar logic to split sections.

Convert everything to ReST without affecting too much
the author's style and avoiding adding uneeded markups.

The conversion is actually:
- add blank lines and identation in order to identify paragraphs;
- fix tables markups;
- add some lists markups;
- mark literal blocks;
- adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

treewide: Add SPDX license identifier - Makefile/Kconfig

Add SPDX license identifiers to all Make/Kconfig files which:

- Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

microblaze: surround string default in Kconfig with double quotes

I guess this is a constant value instead of a symbol.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>

microblaze: Setup dependencies for ASM optimized lib functions

The patch:
"microblaze: Setup proper dependency for optimized lib functions"
(sha1: 7b6ce52be3f86520524711a6f33f3866f9339694)
didn't setup all dependencies properly.
Optimized lib functions in C are also present for little endian
and optimized library functions in assembler are implemented only for
big endian version.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

microblaze: Setup proper dependency for optimized lib functions

Optimized library functions are implemented only for little endian.
Setup this dependency via Kconfig.

commit 206d3642d8ee ("arch/microblaze: add choice for endianness and
update Makefile") added an option to setup endianess via Kconfig.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

microblaze: Remove platform folder

There is no reason to use platform folder structure now.
Everything is OF driven.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>