Allow disabling authentication dynamically

This patch allows platforms to dynamically disable authentication of
images during cold boot. This capability is controlled via the
DYN_DISABLE_AUTH build flag and is only meant for development
purposes.

Change-Id: Ia3df8f898824319bb76d5cc855b5ad6c3d227260
Signed-off-by: Soby Mathew <soby.mathew@arm.com>