doc: move 'Reproducible builds'

Move the README section to the HTML documentation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

README

@@ -1713,17 +1713,6 @@ This firmware often needs to be loaded during U-Boot booting.
 - CONFIG_SYS_MC_RSV_MEM_ALIGN
 	Define alignment of reserved memory MC requires
 
-Reproducible builds
--------------------
-
-In order to achieve reproducible builds, timestamps used in the U-Boot build
-process have to be set to a fixed value.
-
-This is done using the SOURCE_DATE_EPOCH environment variable.
-SOURCE_DATE_EPOCH is to be set on the build host's shell, not as a configuration
-option for U-Boot or an environment variable in U-Boot.
-
-SOURCE_DATE_EPOCH should be set to a number of seconds since the epoch, in UTC.
 
 Building the Software:
 ======================

doc/build/index.rst

@@ -9,6 +9,7 @@ Build U-Boot
    source
    gcc
    clang
+   reproducible
    docker
    tools
    buildman

doc/build/reproducible.rst

+Reproducible builds
+===================
+
+In order to achieve reproducible builds, timestamps used in the U-Boot build
+process have to be set to a fixed value.
+
+This is done using the SOURCE_DATE_EPOCH environment variable which specifies
+the number of seconds since 1970-01-01T00:00:00Z.
+
+Example
+-------
+
+To build the sandbox with 2023-01-01T00:00:00Z as timestamp we can use:
+
+.. code-block:: bash
+
+    make sandbox_defconfig
+    SOURCE_DATE_EPOCH=1672531200 make
+
+This date is shown when we launch U-Boot:
+
+.. code-block:: console
+
+    ./u-boot -T
+    U-Boot 2023.01 (Jan 01 2023 - 00:00:00 +0000)