AP_HAL: use enum for flash_bootloader() result

libraries/AP_HAL/Util.h

@@ -90,8 +90,15 @@ public:
      */
     virtual uint64_t get_hw_rtc() const;
 
+    enum class FlashBootloader {
+        OK=0,
+        NO_CHANGE=1,
+        FAIL=2,
+        NOT_AVAILABLE=3,
+    };
+
     // overwrite bootloader (probably with one from ROMFS)
-    virtual bool flash_bootloader() { return false; }
+    virtual FlashBootloader flash_bootloader() { return FlashBootloader::NOT_AVAILABLE; }
 
     /*
       get system identifier (eg. serial number)