docs/manual: add a new 'Details on Buildroot configuration' section

As discussed with Samuel Martin, there are a few questions about some
Buildroot configuration aspects that come very often on the IRC
channel and on the mailing-list. Topics such as the /dev management
and init systems are often raised.

Therefore, this patch adds a new section that provides details about
the Buildroot configuration possibilities. It does not aim at
replacing the help text of each configuration option, but rather to
complement them by providing more background. Having those
informations in the manual will also make it easier for us to point
newcomers to the right place in the documentation when they have
questions.

Note that this section will replace the "Embedded basics" section that
comes way too late in the manual. This is done in a followup commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>