For the purposes of a VM image, we don't care about autoupdate working. So we could just bump the rootfs size to 4GB

Experimental CL:
https://chromium-review.googlesource.com/c/chromiumos/chromite/+/830671

BUT, this is not good enough because we use the same image to test the provision flow from moblab (we provision the same image onto a "DUT" VM) which will break.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/board-overlays/+/282b6c1671b509869b2821125dc4a56a927d7583

commit 282b6c1671b509869b2821125dc4a56a927d7583
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Tue Dec 19 20:53:15 2017

moblab-generic-vm: Drop overly generic make.defaults from baseboard.

moblab-generic-vm currently uses amd64-generic as the baseboard. This
overlay pulls in too many dependencies because it is a generic board.
That, plus project-moblab dependencies push our rootfs over the 2GB
limit. Drop unnecessary dependencies.

BUG= chromium:795364 
BUG=chromium:775373
TEST=moblab-generic-vm-paladin-tryjob

Change-Id: Iba3b764a2e971e98ca4b7225f4f71b56e33f84ce
Reviewed-on: https://chromium-review.googlesource.com/830908
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>

[add] https://crrev.com/282b6c1671b509869b2821125dc4a56a927d7583/overlay-moblab-generic-vm/profiles/base/make.defaults