manual: move 'Creating your own board support' from User to Developer guide

Section 'Creating your own board support' is seemingly written in the
mindset of adding support for public boards. Therefore, it is more suited in
the Developer guide, rather than in the User guide.
Adding support for custom non-public boards falls under the
'Project-specific customizations' category and will be described in that
section.

This patch moves the unchanged text into a separate file, included from the
Developer guide. The next patch will make some minor changes to the text
itself.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>