manuals: split dev-manual/common-tasks.rst

A 500 KB source file is always harder to manage,
and can have section title conflicts.

So, the "Common Tasks" document is gone and all
its constituents are moved up one level.
You now have 40 chapters in the Development Tasks Manual.

(From yocto-docs rev: 8a45bc469411410020b8e688c137395fcaf3761b)

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

documentation/dev-manual/external-toolchain.rst

@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+Optionally Using an External Toolchain
+**************************************
+
+You might want to use an external toolchain as part of your development.
+If this is the case, the fundamental steps you need to accomplish are as
+follows:
+
+-  Understand where the installed toolchain resides. For cases where you
+   need to build the external toolchain, you would need to take separate
+   steps to build and install the toolchain.
+
+-  Make sure you add the layer that contains the toolchain to your
+   ``bblayers.conf`` file through the
+   :term:`BBLAYERS` variable.
+
+-  Set the ``EXTERNAL_TOOLCHAIN`` variable in your ``local.conf`` file
+   to the location in which you installed the toolchain.
+
+A good example of an external toolchain used with the Yocto Project is
+Mentor Graphics Sourcery G++ Toolchain. You can see information on how
+to use that particular layer in the ``README`` file at
+https://github.com/MentorEmbedded/meta-sourcery/. You can find
+further information by reading about the
+:term:`TCMODE` variable in the Yocto
+Project Reference Manual's variable glossary.
+