binman: Update fit to move node reading into the ReadNode() method

This should not be done in the constructor. Move it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>