binman: Use 'files-compress' to set compression for files

binman: Use 'files-compress' to set compression for files

At present we use 'compress' as the property to set the compression of
a 'files' entry. But this conflicts with the same property for entries,
of which Entry_section is a subclass.

Strictly speaking, since Entry_files is in fact a subclass of
Entry_section, the files can be compressed individually but also the
section (that contains all the files) can itself be compressed. With this
change, it is possible to express that.

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