Add default app start and size limitations

This commit introduces a default application start address (`0x1000`) and size limitation (`0xDF000`) to accomodate the Nordic USB bootloader.

The bootloader consists of a master boot record in flash from address `0x0` to `0x1000` and the actual bootloader application starting at `0xE0000` to the end of flash (`0x100000`). The bootloader enables firmware updates over USB using nRF Connect for Desktop.

More documentation regarding the open bootloader can be found here: https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.0.2/ble_sdk_app_open_bootloader.html

Author: AGlass0fMilk

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_EP_ATLAS/mbed_lib.json

@@ -18,8 +18,9 @@
     },
     "target_overrides": {
         "EP_ATLAS": {
-            "target.network-default-interface-type": "CELLULAR"
-
+            "target.network-default-interface-type": "CELLULAR",
+            "target.mbed_app_start": "0x1000",
+            "target.mbed_app_size": "0xDF000"
         }
 
     }