package/genimage: bump version to 8

This new version fixes segmentation faults on some platforms.

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>