binman: Move GetEntryModules() to control

When binman is installed its main program is in a different directory
to its modules. This means that __file__ is different and we cannot use
it to obtain the path to etype/ from main.py

To fix this, move the function to the 'control' module, since it is
installed with all the other modules, including the etype/ directory.

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