3.8: fix tsl2550 readings in low-light

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
3.8: disable eMMC till capemanager activates it to avoid glitches with capes using those pins

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
3.8: fix LCD7 vsync length

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>