doc: Add documentation for how to build U-Boot host tools

This adds a reST document for how to build U-Boot host tools,
including information for both Linux and Windows.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

doc/build/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Build U-Boot
+============
+
+.. toctree::
+   :maxdepth: 2
+
+   tools

doc/build/tools.rst

@@ -0,0 +1,47 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. sectionauthor:: Bin Meng <bmeng.cn@gmail.com>
+
+Host tools
+==========
+
+Building tools for Linux
+------------------------
+
+To allow distributions to distribute all possible tools in a generic way,
+avoiding the need of specific tools building for each machine, a tools only
+defconfig file is provided.
+
+Using this, we can build the tools by doing::
+
+   $ make tools-only_defconfig
+   $ make tools-only
+
+Building tools for Windows
+--------------------------
+If you wish to generate Windows versions of the utilities in the tools directory
+you can use MSYS2, a software distro and building platform for Windows.
+
+Download the MSYS2 installer from https://www.msys2.org. Make sure you have
+installed all required packages below in order to build these host tools::
+
+   * gcc (9.1.0)
+   * make (4.2.1)
+   * bison (3.4.2)
+   * diffutils (3.7)
+   * openssl-devel (1.1.1.d)
+
+Note the version numbers in these parentheses above are the package versions
+at the time being when writing this document. The MSYS2 installer tested is
+http://repo.msys2.org/distrib/x86_64/msys2-x86_64-20190524.exe.
+
+There are 3 MSYS subsystems installed: MSYS2, MinGW32 and MinGW64. Each
+subsystem provides an environment to build Windows applications. The MSYS2
+environment is for building POSIX compliant software on Windows using an
+emulation layer. The MinGW32/64 subsystems are for building native Windows
+applications using a linux toolchain (gcc, bash, etc), targeting respectively
+32 and 64 bit Windows.
+
+Launch the MSYS2 shell of the MSYS2 environment, and do the following::
+
+   $ make tools-only_defconfig
+   $ make tools-only NO_SDL=1

doc/index.rst

@@ -15,6 +15,17 @@ if you want to help out.
 .. toctree::
    :maxdepth: 2
 
+User-oriented documentation
+---------------------------
+
+The following manuals are written for *users* of the U-Boot - those who are
+trying to get it to work optimally on a given system.
+
+.. toctree::
+   :maxdepth: 2
+
+   build/index
+
 Unified Extensible Firmware (UEFI)
 ----------------------------------