Begin UF2 board-specific family ID

Per @hathach's comments, uses the USB
vendorID and productID.

src/usb/uf2/ghostfat.c

@@ -363,7 +363,13 @@ void read_block(uint32_t block_no, uint8_t *data) {
                 bl->targetAddr = addr;
                 bl->payloadSize = UF2_FIRMWARE_BYTES_PER_SECTOR;
                 bl->flags = UF2_FLAG_FAMILYID;
-                bl->familyID = CFG_UF2_FAMILY_APP_ID;
+
+                bool useBoardId = false; // BUGBUG -- how to detect which familyID to use?
+                if (useBoardId) {
+                  bl->familyID = CFG_UF2_BOARD_APP_ID;
+                } else {
+                  bl->familyID = CFG_UF2_FAMILY_APP_ID;
+                }
                 memcpy(bl->data, (void *)addr, bl->payloadSize);
             }
         }
@@ -390,7 +396,8 @@ int write_block (uint32_t block_no, uint8_t *data, WriteState *state)
 
   switch ( bl->familyID )
   {
-    case CFG_UF2_FAMILY_APP_ID:
+    case CFG_UF2_BOARD_APP_ID:  // board-specific app ... may not be usable on other nrf52 boards
+    case CFG_UF2_FAMILY_APP_ID: // legacy, or where app uses bootloader configuration to discover pins
       /* Upgrading Application
        *
        * SoftDevice is considered as part of application and can be (or not) included in uf2.

src/usb/uf2/uf2cfg.h

@@ -1,9 +1,12 @@
 #include "boards.h"
 #include "dfu_types.h"
 
-// Family ID for updating Application
+// Legacy Family ID for updating Application
 #define CFG_UF2_FAMILY_APP_ID     0xADA52840
 
+// Family ID for board-specific Application
+#define CFG_UF2_BOARD_APP_ID      ((USB_DESC_VID << 16) | USB_DESC_UF2_PID)
+
 // Family ID for updating Bootloader
 #define CFG_UF2_FAMILY_BOOT_ID    0xd663823c