Using the PKI tree with SRKs as intermediate CA isn't necessary or even desirable in some situations (boot time, for example). Add the possibility to use the "fast authentication" method where the image and CSF are both signed using the SRK [1, p.63]. [1] https://community.nxp.com/pwmxy87654/attachments/pwmxy87654/imx-processors/202591/1/CST_UG.pdf Signed-off-by: Brian Ruley <brian.ruley@gehealthcare.com> Cc: Marek Vasut <marex@denx.de> Reviewed-by: Simon Glass <sjg@chromium.org>