Since 'get_repo_dict' does much more than just querying available
repositories, this patch makes it private. The same data is now
available via the 'repo_dict' property. This also means that
'get_repos' is now faster, because it accesses cached data, instead of
calling 'get_repo_dict' that generates this data.
Signed-off-by: Claudius Heine <ch@denx.de>