clearfog: Install RAUC keyring

It seems that a certificate keychain is a required part here.

Change-Id: I25a6fa49c127154b2710ac92a0e2d284fae381cd
diff --git a/configs/czechlight_clearfog_defconfig b/configs/czechlight_clearfog_defconfig
index 7bebbd4..5fdfe43 100644
--- a/configs/czechlight_clearfog_defconfig
+++ b/configs/czechlight_clearfog_defconfig
@@ -12,7 +12,7 @@
 BR2_TARGET_GENERIC_GETTY_BAUDRATE_115200=y
 # BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW is not set
 BR2_ROOTFS_OVERLAY="$(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/common/overlay/ $(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/clearfog/overlay/"
-BR2_ROOTFS_POST_BUILD_SCRIPT="$(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/common/mk-empty-cfg-fs.sh"
+BR2_ROOTFS_POST_BUILD_SCRIPT="$(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/common/mk-empty-cfg-fs.sh $(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/common/install-rauc-cert.sh"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
 BR2_ROOTFS_POST_SCRIPT_ARGS="-c $(BR2_EXTERNAL_CZECHLIGHT_PATH)/board/czechlight/clearfog/genimage.cfg"
 BR2_LINUX_KERNEL=y