Add system/build.prop dependency on vendor/bootimg

When incremental builds only affect the system partition,
the build.prop file for other partitions might go out of
date.

Add a dependency on system/build.prop s.t. when
it's rebuilt,
the fingerprints are updated on the other
partition's build.prop's.

Bug:19058575
Change-Id: Iea7b761e0800c402eb4ce1154e8f6af5a17f7b06