cmake: flash: scripts: Include externally built hex files

Allow user to add externally built hex files to the cmake property
HEX_FILES_TO_MERGE. The hex files in this list will be merged
with the hex file generated when building the zephyr application.

This allows users to leverage the application configuration
available in Kconfig and CMake to help decide what hex file
should be merged with the zephyr application.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>

scripts/mergehex.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+#
+# Copyright (c) 2018 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# This merges a set of input hex files into a single output hex file.
+# Any conflicts will result in an error being reported.
+
+from intelhex import IntelHex
+
+import argparse
+
+
+def merge_hex_files(output, input_hex_files):
+    ih = IntelHex()
+
+    for hex_file_path in input_hex_files:
+        to_merge = IntelHex(hex_file_path)
+
+        # Since 'arm-none-eabi-objcopy' incorrectly inserts record type '03 - Start Segment Address', we need to remove
+        # the start_addr to avoid conflicts when merging.
+        to_merge.start_addr = None
+
+        ih.merge(to_merge)
+    ih.write_hex_file(output)
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(
+        description="Merge hex files.",
+        formatter_class=argparse.RawDescriptionHelpFormatter)
+    parser.add_argument("-o", "--output", required=False, default="merged.hex",
+                        type=argparse.FileType('w', encoding='UTF-8'),
+                        help="Output file name.")
+    parser.add_argument("input_files", nargs='*')
+    return parser.parse_args()
+
+
+def main():
+    args = parse_args()
+
+    merge_hex_files(args.output, args.input_files)
+
+
+if __name__ == "__main__":
+    main()