system: only expose getty options for busybox and sysvinit

system: only expose getty options for busybox and sysvinit

Only busybox and sysvinit handle the BR2_TARGET_GENERIC_GETTY_TERM and
BR2_TARGET_GENERIC_GETTY_OPTIONS options; the other init systems do
not.

So, protect those options behind appropriate dependencies on busybox
or sysvinit.

Fixes #10301.

Reported-by: Michael Heinemann <posted@heine.so>
Suggested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>