Make README more generation even more automated

We have generate-readme-help.sh, but that only generates the output to
put in the code block, it doesn't actually put it there, requiring
something like a visual select and %!./generate-readme-help.sh in vim.
Instead, use some magic comments to mark the region and turn it into a
script that automatically replaces everything between them.

Author: jrtc27

README.md

@@ -292,6 +292,7 @@ I would also suggest using `set autolist` to display all options.
 
 **NOTE:** Since there are so many per-project options that are identical between all projects they are not all shown when running `--help`. To see the full list of options that can be specified, run `cheribuild.py --help-all`. Since this will generate lots of output it probably makes more sense to run `cheribuild.py --help-all | grep <target_name>`.
 
+<!-- BEGIN HELP OUTPUT -->
 ```
 usage: cheribuild.py [-h] [--config-file FILE] [--help-all] [--pretend] [--build] [--test] [--benchmark]
                      [--build-and-test] [--list-targets] [--print-chosen-targets] [--dump-configuration]
@@ -691,3 +692,4 @@ Options for target 'webkit':
                         Use offsets into the JS heap for object references instead of capabilities. This option only
                         affects the purecap backends. (default: 'False')
 ```
+<!-- END HELP OUTPUT -->

generate-readme-help.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+set -e
+set -u
+
+err() {
+	echo >&2 "Error:" "$@"
+	exit 1
+}
+
+if [ ! -f cheribuild.py ] || [ ! -f README.md ]; then
+	err "Not being run in cheribuild's source directory?"
+fi
+
+TEMP=`mktemp`
+exec 3>"$TEMP" 4<"$TEMP"
+rm "$TEMP"
+
+sed -n '
+p
+/<!-- BEGIN HELP OUTPUT -->/q
+' README.md >&3
+
+echo '```' >&3
+PATH="$PWD:$PATH" HOME="\$HOME" XDG_CONFIG_HOME="\$HOME/.config" _GENERATING_README=1 cheribuild.py --help 2>&1 | sed "
+# XXX: Ideally the default would be printed as ~/.config/cheribuild.json rather
+# than the possibly-bundled file.
+s!$PWD/cheribuild.json!\$HOME/.config/cheribuild.json!
+" >&3
+echo '```' >&3
+
+sed -n '
+/<!-- END HELP OUTPUT -->/b loop
+b
+
+: loop
+p
+n
+b loop
+' README.md >&3
+
+cat <&4 >README.md