fit: Verify all configuration signatures

Rather than verifying configuration signature of the configuration node
containing the kernel image types, verify all configuration nodes, even
those that do not contain kernel images. This is useful when the nodes
contain ie. standalone OSes or U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>