Add Checksums to Firmware & External App images (#1809)

* Pad image to 1MB and add simple checksum

* Test code to verify firmware checksum

* Comment out unneeded zlib

* Add files via upload

* Print space remaining in ROM

* Append checksum to external apps too

* Check external app checksums when loading

* Is it 2024 already?!

* Validate firmware checksum before flashing

* Add files via upload

* Added flash error warning to nav screen

* Clang

* Replaced some hard-coded values with #defines

* Check FW checksum before USB serial flash too

* Add files via upload

firmware/tools/simple_checksum.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+
+#
+# Copyright (C) 2024 Mark Thompson
+#
+# This file is part of PortaPack.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+#
+
+import sys
+import struct
+
+usage_message = """
+PortaPack ROM image checksum checker
+
+Usage: <command> <input-file>
+"""
+
+def read_image(path):
+	f = open(path, 'rb')
+	data = f.read()
+	f.close()
+	return data
+
+def write_image(data, path):
+	f = open(path, 'wb')
+	f.write(data)
+	f.close()
+
+if len(sys.argv) != 2:
+	print(usage_message)
+	sys.exit(-1)
+
+image = read_image(sys.argv[1])
+image = bytearray(image)
+
+# simple "add up the words" checksum:
+checksum = 0
+for i in range(0, len(image), 4):
+	checksum += (image[i] + (image[i + 1] << 8) + (image[i + 2] << 16) + (image[i + 3] << 24))
+
+checksum &= 0xFFFFFFFF
+print ("Simple checksum =", checksum)