allow application to implement its own os

src/osal/osal.h

@@ -53,6 +53,8 @@ typedef void (*osal_task_func_t)( void * );
   #include "osal_freertos.h"
 #elif CFG_TUSB_OS == OPT_OS_MYNEWT
   #include "osal_mynewt.h"
+#elif CFG_TUSB_OS == OPT_OS_CUSTOM
+  #include "tusb_os_custom.h" // implemented by application
 #else
   #error OS is not supported yet
 #endif