manual/user guide: move tip on configuration searches

This patch moves the tip on searching in the configuration editors into the
section 'Buildroot configuration', which is more logical.

The wording of that section is slightly adapted to fit this new part.

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