manual: improve documentation about using external toolchains

- Remove the redundant FAQ entry about this topic,
  already covered in a specific section of the Development Tasks manual

- Document the TOOLCHAIN_LOCATION variable

- Expand both the Development Tasks manual and
  the TCMODE variable description using details from the FAQ entry.

- Mention the "meta-arm-toolchain" layer too.

(From yocto-docs rev: 53faa54a8e6311b4d3d41d96cb1a497d0f6502fb)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/faq.rst

@@ -168,23 +168,8 @@ Using the OpenEmbedded Build system
 How do I use an external toolchain?
 -----------------------------------
 
-The toolchain configuration is very flexible and customizable. It
-is primarily controlled with the :term:`TCMODE` variable. This variable
-controls which ``tcmode-*.inc`` file to include from the
-``meta/conf/distro/include`` directory within the :term:`Source Directory`.
-
-The default value of :term:`TCMODE` is "default", which tells the
-OpenEmbedded build system to use its internally built toolchain (i.e.
-``tcmode-default.inc``). However, other patterns are accepted. In
-particular, "external-\*" refers to external toolchains. One example is
-the Sourcery G++ Toolchain. The support for this toolchain resides in
-the separate ``meta-sourcery`` layer at
-https://github.com/MentorEmbedded/meta-sourcery/.
-
-In addition to the toolchain configuration, you also need a
-corresponding toolchain recipe file. This recipe file needs to package
-up any pre-built objects in the toolchain such as ``libgcc``,
-``libstdcc++``, any locales, and ``libc``.
+See the ":ref:`dev-manual/external-toolchain:optionally using an external toolchain`"
+section in the Development Task manual.
 
 Why do I get chmod permission issues?
 -------------------------------------

documentation/ref-manual/variables.rst

@@ -2417,6 +2417,12 @@ system and gives an overview of their function and contents.
       :ref:`kernel-yocto <ref-classes-kernel-yocto>` class in
       ``meta/classes-recipe`` to see how the variable is used.
 
+   :term:`EXTERNAL_TOOLCHAIN`
+      When you intend to use an
+      :ref:`external toolchain <dev-manual/external-toolchain:optionally using an external toolchain>`,
+      this variable allows to specify the directory where this toolchain was
+      installed.
+
    :term:`EXTERNALSRC`
       When inheriting the :ref:`externalsrc <ref-classes-externalsrc>`
       class, this variable points to the source tree, which is outside of
@@ -8475,16 +8481,20 @@ system and gives an overview of their function and contents.
       https://github.com/MentorEmbedded/meta-sourcery/.
 
       The layer's ``README`` file contains information on how to use the
-      Sourcery G++ Toolchain as an external toolchain. In summary, you must
-      be sure to add the layer to your ``bblayers.conf`` file in front of
-      the ``meta`` layer and then set the ``EXTERNAL_TOOLCHAIN`` variable
-      in your ``local.conf`` file to the location in which you installed
-      the toolchain.
+      Sourcery G++ Toolchain as an external toolchain. You will have to
+      add the layer to your ``bblayers.conf`` file and then set the
+      :term:`EXTERNAL_TOOLCHAIN` variable in your ``local.conf`` file to
+      the location of the toolchain.
 
       The fundamentals used for this example apply to any external
       toolchain. You can use ``meta-sourcery`` as a template for adding
       support for other external toolchains.
 
+      In addition to toolchain configuration, you will also need a
+      corresponding toolchain recipe file. This recipe file needs to package
+      up any pre-built objects in the toolchain such as ``libgcc``,
+      ``libstdcc++``, any locales, and ``libc``.
+
    :term:`TC_CXX_RUNTIME`
       Specifies the C/C++ STL and runtime variant to use during
       the build process. Default value is 'gnu'