Add support for configurable stack size

[motgon01]

Description

• Type: Enhancement
• Priority: Major

Enhancement

Reason to enhance or problem with existing solution
There is no way to configure application's main thread stack size at build time.

Suggested enhancement
Add stack_size attribute to mbed_app.json JSON file that will set the initial main thread stack size.

@jenia81

[bulislaw]

That's sort of configured per target (kind of anyway). We are in process of upgrading RTX to v2, I think that's a good idea, but whether we use default from target and overwrite it if the change is requested in mbed_app.json or assume default main stack size and let people overwrite it in mbed_app.json or maybe we could have one, default configuration that can be overwritten if needed needs to be decided. #3218 #2984

[nuket]

There were some hints about this here:
#2635 (comment)

GitHib issue review: Closed due to inactivity. Please re-file if critical issues found.