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/compile-fletcher16-gen.sh

@@ -0,0 +1,42 @@
+#!/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