[cmake] add `OT_BORDER_AGENT_EPSKC` in CMake options (#10034)

Tied to `OPENTHREAD_CONFIG_BORDER_AGENT_EPHEMERAL_KEY_ENABLE`.
diff --git a/etc/cmake/options.cmake b/etc/cmake/options.cmake
index 87b57da..eb41ec8 100644
--- a/etc/cmake/options.cmake
+++ b/etc/cmake/options.cmake
@@ -174,6 +174,7 @@
 ot_option(OT_BACKBONE_ROUTER_MULTICAST_ROUTING OPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE "BBR MR")
 ot_option(OT_BLE_TCAT OPENTHREAD_CONFIG_BLE_TCAT_ENABLE "Ble based thread commissioning")
 ot_option(OT_BORDER_AGENT OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE "border agent")
+ot_option(OT_BORDER_AGENT_EPSKC OPENTHREAD_CONFIG_BORDER_AGENT_EPHEMERAL_KEY_ENABLE "border agent ephemeral PSKc")
 ot_option(OT_BORDER_AGENT_ID OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE "create and save border agent ID")
 ot_option(OT_BORDER_ROUTER OPENTHREAD_CONFIG_BORDER_ROUTER_ENABLE "border router")
 ot_option(OT_BORDER_ROUTING OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE "border routing")