meta-aspeed: u-boot-aspeed-sdk: Use provided signing key by default

Allow configurations to override the SPL signing key if necessary, but
provide a default in case they don't. The provided key is intended a
low-security key.

Change-Id: I756271970b0a55409e40dbd06f1245ada6a3cb73
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>