binman: Allow easy importing of entry modules

binman: Allow easy importing of entry modules

At present entry modules can only be accessed using Entry.Lookup() or
Entry.Create(). Most of the time this is fine, but sometimes a module
needs to provide constants or helper functions useful to other modules.
It is easier in this case to use 'import'.

Add an __init__ file to permit this.

Signed-off-by: Simon Glass <sjg@chromium.org>