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.

(From OE-Core rev: 62269642ce0e0e56d68d495b6c4d27327c9ed649)

Signed-off-by: Leandro Dorileo <ldorileo@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/combo-layer

@@ -79,6 +79,14 @@ local_repo_dir = ~/src/oecore
 dest_dir = .
 last_revision =
 
+# it's also possible to embed python code in the config values. Similar
+# to bitbake it considers every value starting with @ to be a python script. 
+# So local_repo could be easily configured using an environment variable as:
+#
+# [bitbake]
+# local_repo = @os.getenv("LOCAL_REPO_DIR") + "/bitbake"
+#
+
 # more components ...
 
     """
@@ -91,7 +99,10 @@ last_revision =
         for repo in self.parser.sections():
             self.repos[repo] = {}
             for (name, value) in self.parser.items(repo):
-                self.repos[repo][name] = value
+                if value.startswith("@"):
+                    self.repos[repo][name] = eval(value.strip("@"))
+                else:
+                    self.repos[repo][name] = value
 
     def update(self, repo, option, value):
         self.parser.set(repo, option, value)