binman: Move 'external' support into base class

At present we have an Entry_blob_ext which implement a blob which holds an
external binary. We need to support other entry types that hold external
binaries, e.g. Entry_blob_named_by_arg. Move the support into the base
Entry class to allow this.

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