binman: add fast authentication method for i.MX8M signing

binman: add fast authentication method for i.MX8M signing

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>