mkimage: Add -K to write public keys to an FDT blob

FIT image verification requires public keys. Add a convenient option to
mkimage to write the public keys to an FDT blob when it uses then for
signing an image. This allows us to use:

   mkimage -f test.its -K dest.dtb -k keys test.fit

and have the signatures written to test.fit and the corresponding public
keys written to dest.dtb. Then dest.dtb can be used as the control FDT
for U-Boot (CONFIG_OF_CONTROL), thus providing U-Boot with access to the
public keys it needs.

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