BL NUCLEO_F746ZG: enable bootloader

Author: bcostm

targets/targets.json

@@ -1289,7 +1289,8 @@
         "device_has_add": ["ANALOGOUT", "CAN", "LOWPOWERTIMER", "SERIAL_ASYNCH", "TRNG", "FLASH"],
         "features": ["LWIP"],
         "release_versions": ["2", "5"],
-        "device_name": "STM32F746ZG"
+        "device_name": "STM32F746ZG",
+        "bootloader_supported": true
     },
     "NUCLEO_F756ZG": {
         "inherits": ["FAMILY_STM32"],