Add example keys for building trusted flash images using
doimage tools.
Similar files can be generated using openssl or mbedtls.
Marvell platform make files are using trusted boot
configurations from this example etst vector.

Change-Id: I38a2e295171bee4c14005ce6f020b352c683496e
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>