docs/manual: add genimage.cfg file coding style standard

This patch adds the explanation for the genimage.cfg files coding standard.
A real standard is not defined upstream by the genimage utility
developers[1], but I try to give one here in Buildroot for consistency.

[1]: https://github.com/pengutronix/genimage



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>