config: Don't synchronize user-provided sourcedir

To reuse an existing repository, the recipes.rst documentation suggests overriding the 'sourcedir' variable. However, this will attempt to synchronize the user source tree from the existing 'repo' configuration, overriding any changes the user made.

Clear the 'repo' configuration when the user provides their own source tree.

Signed-off-by: Jean-Philippe Brucker jean-philippe@linaro.org