Add content of the mkpkg_layout man page
Add default config as $config->{default}

Don't merge default config in the main config, and instead make it
available as $config->{default}. In some cases (like next commit) we
may need to access both the main and default config, which is not
possible when they have been merged.
project_config: add the as_array option

Add the 'as_array' option to the project_config function, which will
return all matching results as an array instead of only the first one.
Add support for distributions configuration

Add automatic detection of running distribution using lsb_release.

Add support for distributions configuration. It's now possible to
define options depending on the selected distributions.