lib/rsa: Use the 'keyfile' argument from mkimage

Keys can be derived from keydir, and the "key-name-hint" property of
the FIT. They can also be specified ad-literam via 'keyfile'. Update
the RSA signing path to use the appropriate one.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>