Increasing sysimage partition size for all boards

This change fixes issue b/25613506

The predefined, fixed system image partition size is failing
to fit content for NYC release MIPS64 images. This change
increases the system image size for all boards to 1.5GB
(up from 1.25GB) to make sure that the system image sizes are
uniform across all virtual boards, and fit new content.

Change-Id: Id9808ad5318cd2390fc666ac35b0f9cd32870993