Add device id.

It's available from the serial too but I'd like to move away from that
for version identification too now we know that Brave randomises it.

lib/constants.ts

@@ -15,6 +15,8 @@
 export const FICR = {
   CODEPAGESIZE: 0x10000000 | 0x10,
   CODESIZE: 0x10000000 | 0x14,
+
+  DEVICE_ID_1: 0x10000000 | 0x64,
 };
 
 export const DapCmd = {

lib/usb-device-wrapper.ts

@@ -28,6 +28,7 @@ export class DAPWrapper {
 
   _pageSize: number | undefined;
   _numPages: number | undefined;
+  _deviceId: number | undefined;
 
   private loggedBoardSerialInfo: BoardSerialInfo | undefined;
 
@@ -62,6 +63,13 @@ export class DAPWrapper {
     return this._numPages;
   }
 
+  /**
+   * The number of pages. Undefined if we've not connected.
+   */
+  get deviceId() {
+    return this._deviceId;
+  }
+
   get boardSerialInfo(): BoardSerialInfo {
     return BoardSerialInfo.parse(
       this.device,
@@ -112,6 +120,8 @@ export class DAPWrapper {
 
     this._pageSize = await this.cortexM.readMem32(FICR.CODEPAGESIZE);
     this._numPages = await this.cortexM.readMem32(FICR.CODESIZE);
+    // https://support.microbit.org/support/solutions/articles/19000067679-how-to-find-the-name-of-your-micro-bit
+    this._deviceId = await this.cortexM.readMem32(FICR.DEVICE_ID_1);
   }
 
   async startSerial(listener: (data: string) => void): Promise<void> {

lib/usb.ts

@@ -196,6 +196,10 @@ export class MicrobitWebUSBConnection
     });
   }
 
+  getDeviceId(): number | undefined {
+    return this.connection?.deviceId;
+  }
+
   getBoardVersion(): BoardVersion | undefined {
     return this.connection?.boardSerialInfo?.id.toBoardVersion();
   }