Fix build failure when target's cmsis doesn't exist

For bootloader enabled target, it needs to have cmsis pack or provide memory override. This change fixes build failure in case of no cmsis pack but providing memory override with start being zero.

tools/config/__init__.py

@@ -798,12 +798,12 @@ class Config(object):
             start, size = self._get_primary_memory_override(
                 active_memory.lower()
             )
-            if not start:
+            if start is None:
                 raise ConfigException(
                     "Bootloader not supported on this target. {} "
                     "start not found in targets.json.".format(active_memory)
                 )
-            if not size:
+            if size is None:
                 raise ConfigException(
                     "Bootloader not supported on this target. {} "
                     "size not found in targets.json.".format(active_memory)