image-dsk.bbclass: require Secure Boot key if and only if feature enabled
image-dsk.bbclass: require Secure Boot key if and only if feature enabled

Instead of erroring out at parse time when files are not found, only
do that check at build time when the feature is actually enabled. That
makes it easier to use the class without Secure Boot signing.

Signed-off-by: Patrick Ohly <0fc9d825c622b2a5ac5160289c29f03654b10648@intel.com>

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