gateway: working shell on uart0 and mcumgr on usb_serial, minimal OS mgmt config\n

Signed-off-by: Eduard Iten <eduard@iten.pro>

software/apps/gateway/sysbuild/gateway.overlay

@@ -0,0 +1,12 @@
+#include "../boards/common_4MB.dtsi"
+
+/* Application Configuration - Firmware goes to slot0_partition (0x20000) */
+/ {
+    chosen {
+        zephyr,code-partition = &slot0_partition;
+    };
+};
+
+&usb_serial {
+    status = "okay";
+};