Merge pull request ARMmbed#30 from bentcooke/NUCLEO_L476RG_add

Add NUCLEO_L476RG bootloader support config

Author: LiyouZhou

mbed_app.json

@@ -38,6 +38,11 @@
             "update-client.application-details": "0x08020000",
             "firmware_metadata_header_size": "0x400"
         },
+        "NUCLEO_L476RG": {
+            "firmware_metadata_header_address": "0x08020000",
+            "update-client.application-details": "0x08020000",
+            "firmware_metadata_header_size": "0x800"
+        },
         "UBLOX_EVK_ODIN_W2": {
             "firmware_metadata_header_address": "0x08020000",
             "update-client.application-details": "0x08020000",