Increase system image partition size to fix build

Build of sdk_google_phone_x86_64-sdk_addon product was failing with
the following error message:

error: ext4_allocate_best_fit_partial: failed to allocate 2420 blocks,
out of space?

Change-Id: Ib9e2d21bac86b12b6f8f75d4f30806dd20abcb90