scripts/pnmtologo: fix for plain PBM

PBM generated with current tools do not have a whitespace between the
digits. Therefore the pnmtologo tool fails to gernerate the required
C-Array for these images. This patch fixes that behaviour and can
handle both 'old style' and 'new style' PBM files.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>