phase1: add JSON merge step

phase1: add JSON merge step

The refactored JSON info files require a final merge step which sums up
all created JSON info files of a target into a single `profiles.json`
files.

This patch adds the extra step to run `make json_overview_image_info`
just before calculating the checksums so the `profiles.json` files is
signed as well.

Signed-off-by: Paul Spooren <[email protected]>
[allow the step to fail to satisfy 18.06 builders]
Signed-off-by: Jo-Philipp Wich <[email protected]>