Read OS env config vars as setup phase 110

[uwiger]

See #3298
Adding a setup start phase 110 (to be run immediately after 100 where the user config is read), OS environment variables are checked for custom config. The keys looked for start with AECONF__, followed by the key parts separated by two underscores (__). The reason for this choice was that the alphabet one can expect to work in all shells contains alphanumerics and underscores only (in POSIX, env var names can contain anything except =, I believe). While AE config variable names do contain underscores, none of them contains two in a row.

Example:

AECONF__chain__persist=true AECONF__chain__db_path=~/ae/5/aeternity/_build/test/logs/latest.fork/dev1/data/mnesia make console

• No test cases yet.
• Type coercion is done for integers, strings and booleans. Arrays not yet handled.