clearfog: Install RAUC keyring

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

Change-Id: I25a6fa49c127154b2710ac92a0e2d284fae381cd
diff --git a/board/czechlight/clearfog/overlay/etc/rauc/system.conf b/board/czechlight/clearfog/overlay/etc/rauc/system.conf
index 8adc7f9..8fc94d4 100644
--- a/board/czechlight/clearfog/overlay/etc/rauc/system.conf
+++ b/board/czechlight/clearfog/overlay/etc/rauc/system.conf
@@ -2,6 +2,9 @@
 compatible=czechlight-clearfog
 bootloader=uboot
 
+[keyring]
+path=/etc/rauc/keyring.pem
+
 [slot.rootfs.0]
 device=/dev/mmcblk0p1
 type=ext4