mkimage: Add -r option to specify keys that must be verified

Normally, multiple public keys can be provided and U-Boot is not
required to use all of them for verification. This is because some
images may not be signed, or may be optionally signed.

But we still need a mechanism to determine when a key must be used.
This feature cannot be implemented in the FIT itself, since anyone
could change it to mark a key as optional. The requirement for
key verification must go in with the public keys, in a place that
is protected from modification.

Add a -r option which tells mkimage to mark all keys that it uses
for signing as 'required'.

If some keys are optional and some are required, run mkimage several
times (perhaps with different key directories if some keys are very
secret) using the -F flag to update an existing FIT.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

doc/mkimage.1

@@ -133,6 +133,12 @@ the corresponding public key is written into this file for for run-time
 verification. Typically the file here is the device tree binary used by
 CONFIG_OF_CONTROL in U-Boot.
 
+.TP
+.BI "\-r
+Specifies that keys used to sign the FIT are required. This means that they
+must be verified for the image to boot. Without this option, the verification
+will be optional (useful for testing but not for release).
+
 .SH EXAMPLES
 
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