Add fletcher16-gen developer util

1. I'm not convinced our fletcher16 implementation is proper
2. It is not in coreutils

So we add some basic code to do that.

This is also the first dev-util, so add some documentation to DEVEL.md.

dev-utils/fletcher16-gen/.gitignore

@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>
+#
+# SPDX-License-Identifier: MIT
+
+fletcher16.M1
+fletcher16-footer.M1
+fletcher16.hex2
+fletcher16-gen

dev-utils/fletcher16-gen/compile-fletcher16-gen.sh

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+#!/bin/sh
+
+# SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set -ex
+
+M2-Planet \
+    --architecture x86 \
+    -f ../m2-functions/in_set.c \
+    -f ../m2-functions/file_print.c \
+    -f ../m2-functions/numerate_number.c \
+    -f ../m2-functions/string.c \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/functions/file.c \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/functions/exit.c \
+    -f ../m2-functions/require.c \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/functions/malloc.c \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/functions/calloc.c \
+    -f fletcher16-gen.c \
+    -o fletcher16.M1 \
+    --debug
+
+blood-elf -f fletcher16.M1 -o fletcher16-footer.M1
+
+M1 \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/x86_defs.M1 \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/libc-core.M1 \
+    -f fletcher16.M1 \
+    -f fletcher16-footer.M1 \
+    --LittleEndian \
+    --architecture x86 \
+    -o fletcher16.hex2
+
+hex2 \
+    -f ../../sysa/mescc-tools-seed/src/mescc-tools-seed/M2-Planet/test/common_x86/ELF-i386-debug.hex2 \
+    -f fletcher16.hex2 \
+    --LittleEndian \
+    --architecture x86 \
+    --BaseAddress 0x8048000 \
+    -o fletcher16-gen \
+    --exec_enable

dev-utils/fletcher16-gen/fletcher16-gen.c

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+/*
+ * SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+
+#define BUF_SIZE 1024
+// CONSTANT BUF_SIZE 1024
+#define PATH_MAX 512
+// CONSTANT PATH_MAX 512
+
+int fletch16(FILE* fp)
+{
+	long sum1 = 0;
+	long sum2 = 0;
+	int index;
+	int c = fgetc(fp);
+	while(c != EOF)
+	{
+		sum1 = (sum1 + c) % 255;
+		sum2 = (sum2 + sum1) % 255;
+		c = fgetc(fp);
+	}
+
+	return (sum2 << 8) | sum1;
+}
+
+int main(int argc, char** argv)
+{
+	/* Open the file containing the checksums */
+	FILE* fp = fopen(argv[1], "r");
+	require(fp != NULL, prepend_string(
+			prepend_string("Error opening checksum file ", argv[1]), "\n"));
+	int csum = fletch16(fp);
+	file_print(prepend_string(prepend_string(prepend_string(
+				numerate_number(csum), " "), argv[1]), "\n"), stdout);
+}