binman: Move compression into the Entry base class

binman: Move compression into the Entry base class

Compression is currently available only with blobs. However we want to
report the compression algorithm and uncompressed size for all entries,
so that other entry types can support compression. This will help with
the forthcoming 'list' feature which lists entries in the image.

Move the compression properties into the base class. Also fix up the docs
which had the wrong property name.

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