next-cube.c: move static led variable to NeXTPC

next-cube.c: move static led variable to NeXTPC

The state of the led is stored in the SCR2 register which is part of the NeXTPC
device.

Note that this is a migration break for the NeXTPC device, but as nothing will
currently boot then we simply bump the migration version for now.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20231220131641.592826-7-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>