mambo: Add more tweakable security flags
mambo: Add more tweakable security flags

This allows us to test various combinations of the Linux
spectre/meltdown mitigations.

The default values of the existing settings remain the same.

Signed-off-by: Michael Ellerman <864f124608374e06e4da1fa5d2e47ed839b95411@ellerman.id.au>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

legoater/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot