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>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 97bfae1..1086a6a 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -692,7 +692,7 @@
         when writing out the map
 
 Properties:
-    _allow_missing: True if this section permits external blobs to be
+    allow_missing: True if this section permits external blobs to be
         missing their contents. The second will produce an image but of
         course it will not work.