binman: Don't add compression attribute for uncompressed files

cbfsutil changed to skip adding a compression attribute if there is no
compression. Adjust the binman implementation to do the same.

This mirrors commit 105cdf5625 in coreboot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>