scripts/combo-layer: a simple way to script the combo-layer conf

scripts/combo-layer: a simple way to script the combo-layer conf

This small patch introduces a a very simple and basic way to script
the combo-layer conf file. With that a combo can be shared with no
need to change its config - associated to the use of environment
variables for example.

*Similar* to bitbake it considers every value starting with @ to be
a python script. So local_repo could be easily configured as:

[bitbake]
local_repo = @os.getenv("LOCAL_REPO_DIR") + "/bitbake"

or any more sophisticated python syntax.

This version updates the config file description so users can be
aware of.

Signed-off-by: Leandro Dorileo <ldorileo@gmail.com>