config.logging.persistent enables /dev/pmsg0

- whatever's written to /dev/pmsg0 appears as
/sys/fs/pstore/pmsg-ramoops-0 after reboot

- only works on devices with the relevant device tree
support (gl-ar750 and whatever has it by default)

- nothing in the system is actually writing this file yet

- or reading it at boot time, for that matter

devices/gl-ar750/pstore-ramoops.dtsi

@@ -0,0 +1,10 @@
+/ {
+reserved-memory {
+		ramoops@03f00000 {
+			compatible = "ramoops";
+			reg = <0x03f00000 0x10000>;
+			record-size = <0x1000>;
+			pmsg-size = <0x1000>;
+		};
+	};
+};