cmake: fix evaluation with empty signature key file
cmake: fix evaluation with empty signature key file

When CONFIG_BOOT_SIGNATURE_KEY_FILE is left empty, the cmake find command gets
called with a wrong number of arguments. This patch skips the search for the
signature key file within config files in that case.

Signed-off-by: Nico Lastzka <3b41057d1c476684df27d40da6fe4627248b3fed@ATSonline.de>

runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot