doc: move out-of-tree building info to HTML

Move the information about out-of-tree building
from README to the generated HTML documentation.

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

1 files changed, 28 insertions, 0 deletions

diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index 3c64657..d8fcfdc 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -118,6 +118,34 @@ Assuming cross compiling on Debian for ARMv8 this would be
 
     CROSS_COMPILE=aarch64-linux-gnu- make
 
+Out-of-tree building
+~~~~~~~~~~~~~~~~~~~~
+
+By default building is performed locally and the objects are saved in the source
+directory. To build out-out-tree use one of the two methods below:
+
+Add O= parameter to the make command line:
+
+.. code-block:: bash
+
+    make O=/tmp/build distclean
+    make O=/tmp/build NAME_defconfig
+    make O=/tmp/build
+
+Use environment variable KBUILD_OUTPUT:
+
+.. code-block:: bash
+
+    export KBUILD_OUTPUT=/tmp/build
+    make distclean
+    make NAME_defconfig
+    make
+
+.. note::
+
+    The command line "O=" parameter overrides the KBUILD_OUTPUT environment
+    variable.
+
 Build parameters
 ~~~~~~~~~~~~~~~~