Add a config test to check for failing bootloader images

Author: theotherjimmy

tools/test/config_test/test29/mbed_app.json

@@ -0,0 +1,7 @@
+{
+    "target_overrides": {
+        "K64F": {
+            "target.bootloader_img": "does_not_exists.bin"
+        }
+    }
+}

tools/test/config_test/test29/test_data.py

@@ -0,0 +1,6 @@
+expected_results = {
+    "K64F": {
+        "desc": "error when bootloader not found",
+        "exception_msg": "not found"
+    }
+}