chibios_hwdef.py: allow re-use of bootloader from other boards

libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py

@@ -2408,12 +2408,19 @@ def bootloader_path(self):
         this_dir = os.path.realpath(__file__)
         rootdir = os.path.relpath(os.path.join(this_dir, "../../../../.."))
         hwdef_dirname = os.path.basename(os.path.dirname(args.hwdef[0]))
+        # allow re-using of bootloader from different build:
+        use_bootloader_from_board = self.get_config('USE_BOOTLOADER_FROM_BOARD', default=None, required=False)
+        if use_bootloader_from_board is not None:
+            hwdef_dirname = use_bootloader_from_board
         bootloader_filename = "%s_bl.bin" % (hwdef_dirname,)
         bootloader_path = os.path.join(rootdir,
-                                       "Tools",
-                                       "bootloaders",
-                                       bootloader_filename)
-        return bootloader_path
+                                    "Tools",
+                                    "bootloaders",
+                                    bootloader_filename)
+        if os.path.exists(bootloader_path):
+            return os.path.realpath(bootloader_path)
+
+        return None
 
     def embed_bootloader(self, f):
         '''added bootloader to ROMFS'''