docs(rss): update RSS doc for signer-ID

Added details about the API that calculates the signer-ID and updated
console log details to provide signer-ID information for each image.

Change-Id: If637b3719418e9c0b8d2844c92bddbdfe454bfb8
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed