Extend lc_open() to also being able to load overlay plugins.

For this it needs to look at the plugin header. Since lc_open() doesn't know
it's a plugin, the header needs to be changed slightly to include the new lc_header (which needs to be the first element in plugin_header so it can be casted savely).

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28054 a1c6a512-1295-4272-9138-f99709370657
10 files changed