In preparation of more section and examples to multiconfig, move the
section about it in building.rst into its own document.
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 6f60fb09976540dd320816684684c14f6b7ab460)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>