binman: Convert existing binary blobs to blob_ext

binman: Convert existing binary blobs to blob_ext

Many of the existing blobs rely on external binaries which may not be
available. Move them over to use blob_ext to indicate this.

Unfortunately cros-ec-rw cannot use this class because it inherits
another. So set the 'external' value for that class.

While we are here, drop the import of Entry since it is not used (and
pylint3 complains).

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