Move fsverity metadata generation to Makefile

build_image.py has been handling fsverity metadata generation in the
packing step, but it can cause issues because the metadata files are
missing in the $OUT directory, and they only exist in result system.img.
This change moves the generation logic into Makefile, and makes the
metadata tracked by ninja graph.

Bug: 206326351
Test: PRODUCT_SYSTEM_FSVERITY_GENERATE_METADATA := true and build
Change-Id: I1f910d8ac6e2cc3c54f35916871733c632f18e44