Revert "Remove -nostartfiles linker option from bootloader (#5996)"

This reverts commit 17b4aa2466.

Removing the -nostartfiles caused the Taranis bootloader to include
a number of strange functions. Instead define _fini to avoid the linker
issues.

radio/src/targets/common/arm/stm32/bootloader/boot.cpp

@@ -526,4 +526,8 @@ int main()
   }
 
   return 0;
-}
+}
+
+#if defined(PCBHORUS)
+void *__dso_handle = 0;
+#endif