board/roseapplepi/genimage.cfg: beautify file

Beatify this genimage .cfg file to have consistency with all genimage .cfg
files in Buildroot.

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