Ensure target bootloader settings can be overridden in mbed_app.json

Values like `target.mbed_rom_start` were being replaced by None and then ignored.

Author: pi-anl

tools/config/__init__.py

@@ -495,7 +495,8 @@ def __init__(self, tgt, top_level_dirs=None, app_config=None):
         self.target_labels = self.target.labels
         for override in BOOTLOADER_OVERRIDES:
             _, attr = override.split(".")
-            setattr(self.target, attr, None)
+            if not hasattr(self.target, attr):
+                setattr(self.target, attr, None)
 
         self.cumulative_overrides = {key: ConfigCumulativeOverride(key)
                                      for key in CUMULATIVE_ATTRIBUTES}