Proposing update_env_if construct to CM script meta

[arjunsuresh]

Currently CM script meta is not having an option to update env variables based on some conditions and these updates need to happen in customize.py. This works fine for small scripts but for generic scripts like get-generic-python-lib this increases the complexity of the code in customize.py.

Any deps can update a ENV variables - for example detect-cpu can set CPU_ARCHITECTURE as x86 and if get-generic-sys-util has selected a package say X we can update the installation source for X on x86 in _cm.yml itself if we can have the below construct.

update_env_if: {
  "env": {
  ....
  }
  "skip_if_env": {
  ....
  } 
}

Here, we can use skip_if_env, enable_if_env, skip_if_any_env and enable_if_any_env which we currently use to conditionally turn on dependencies.