use key-value format in stage0 file

Currently, we are working on the python removal task on bootstrap. Which means
we have to extract some data from the stage0 file using shell scripts. However,
parsing values from the stage0.json file is painful because shell scripts don't
have a built-in way to parse json files.

This change simplifies the stage0 file format to key-value pairs, which makes
it easily readable from any environment.

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan