Consider using system_a and system_b in LVM

[rinigus]

It is possible to add system_a and system_b into LVM pool. This would increase storage by the part that we don't really use at all right now (system_a) and by using rarely used feature (factory reset on device).

Corresponding changes have to be done in

• https://github.com/sailfishos-sony-tama/hybris-initrd/blob/master/sbin/root-mount
• https://github.com/sailfishos-sony-tama/droid-config-sony-tama-pie

Space available:

• system_a: 4GB
• system_b: 4GB