tls: force write certificates to disk

On embedded devices such as a RPI storage is more fragile and power loss
can happen more frequently.

Co-Authored-By: Jean-Marie Lemetayer <j.lemetayer@kerlink.fr>

src/tls/cockpit-certificate-helper.in

@@ -14,11 +14,17 @@ install_cert() {
 
     # The certificate should be world-readable
     chmod a+r "${destination}"
+
+    # Force flush to disk for embedded devices
+    sync "${destination}"
 }
 
 install_key() {
     local destination="${COCKPIT_WS_CERTS_D}/$1"
     mv -Z "$1" "${destination}"
+
+    # Force flush to disk for embedded devices
+    sync "${destination}"
 }
 
 selfsign_sscg() {