Revert "always use/define CFG_UF2_BOARD_APP_ID"

This reverts commit aa8cd83,
because the feather_nrf52832 does not define its own USB VID/PID.

Author: henrygab

src/usb/uf2/ghostfat.c

@@ -363,7 +363,12 @@ 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;
+
+#if defined(CFG_UF2_BOARD_APP_ID) && CFG_UF2_BOARD_APP_ID
                 bl->familyID = CFG_UF2_BOARD_APP_ID;
+#else
+                bl->familyID = CFG_UF2_FAMILY_APP_ID;
+#endif
                 memcpy(bl->data, (void *)addr, bl->payloadSize);
             }
         }
@@ -390,8 +395,12 @@ int write_block (uint32_t block_no, uint8_t *data, WriteState *state)
 
   switch ( bl->familyID )
   {
+
+#if defined(CFG_UF2_BOARD_APP_ID) && CFG_UF2_BOARD_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: // family-specific app ... Legacy apps, or where the app uses bootloader configuration to discover pins
+#endif
+
+    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

@@ -1,15 +1,13 @@
 #include "boards.h"
 #include "dfu_types.h"
 
-#if !defined(USB_DESC_VID) || !defined(USB_DESC_UF2_PID) || !USB_DESC_VID || !USB_DESC_UF2_PID
-    #error "Per @hathach, 'This repo doesn't accept nrf52840/833 that doesn't have VID/PID'"
-#endif
-
-
-// Family ID for updating Application
+// Legacy Family ID for updating Application
 #define CFG_UF2_FAMILY_APP_ID     0xADA52840
-// Board ID for board-specific Application
-#define CFG_UF2_BOARD_APP_ID      ((USB_DESC_VID << 16) | USB_DESC_UF2_PID)
+
+// Family ID for board-specific Application
+#if defined(USB_DESC_VID) && defined(USB_DESC_UF2_PID) && USB_DESC_VID && USB_DESC_UF2_PID
+    #define CFG_UF2_BOARD_APP_ID      ((USB_DESC_VID << 16) | USB_DESC_UF2_PID)
+#endif
 
 
 // Family ID for updating Bootloader