mkimage: Add -F option to modify an existing .fit file

When signing images it is sometimes necessary to sign with different keys
at different times, or make the signer entirely separate from the FIT
creation to avoid needing the private keys to be publicly available in
the system.

Add a -F option so that key signing can be a separate step, and possibly
done multiple times as different keys are avaiable.

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

doc/mkimage.1

@@ -9,6 +9,9 @@ mkimage \- Generate image for U-Boot
 .B mkimage
 .RB [\fIoptions\fP] " \-f [" "image tree source file" "]" " [" "uimage file name" "]"
 
+.B mkimage
+.RB [\fIoptions\fP] " \-F [" "uimage file name" "]"
+
 .B mkimage
 .RB [\fIoptions\fP] " (legacy mode)"
 
@@ -103,6 +106,13 @@ create the image.
 Image tree source file that describes the structure and contents of the
 FIT image.
 
+.TP
+.BI "\-F"
+Indicates that an existing FIT image should be modified. No dtc
+compilation is performed and the -f flag should not be given.
+This can be used to sign images with additional keys after initial image
+creation.
+
 .TP
 .BI "\-k [" "key_directory" "]"
 Specifies the directory containing keys to use for signing. This directory
@@ -144,6 +154,16 @@ skipping those for which keys cannot be found. Also add a comment.
 -c "Kernel 3.8 image for production devices" kernel.itb
 .fi
 
+.P
+Update an existing FIT image, signing it with additional keys.
+Add corresponding public keys into u-boot.dtb. This will resign all images
+with keys that are available in the new directory. Images that request signing
+with unavailable keys are skipped.
+.nf
+.B mkimage -F -k /secret/signing-keys -K u-boot.dtb \\\\
+-c "Kernel 3.8 image for production devices" kernel.itb
+.fi
+
 .SH HOMEPAGE
 http://www.denx.de/wiki/U-Boot/WebHome
 .PP