[images] Write a max allowable FVM size to build dir base on variable

This adds a build step that generates a file containing a maximum
allowable FVM size limit. The limit can be configured from product
configurations and/or specified as a GN argument.

Test: build with this configuration
INTK-773 #comment add build infrastructure

Change-Id: I9c52975d7660df154924c813ba7587b96aa2f27d
2 files changed