Add new helper script for repo.turris.cz relative turris-build checkout

The idea is to have script that does the same thing as we do for
releases but for not yet released builds.
Usage is mostly for automated builds based on our repository.
Successful build on our servers somewhat ensures also successful build
anywhere else. Building latest successful version takes users to success
more likely than just building tip of git branch.

helpers/checkout_repo_branch.sh

+#!/bin/bash
+set -o errexit -o nounset
+
+helpers_dir="$(dirname "$(readlink -f "$0")")"
+. "$helpers_dir/common.sh"
+
+print_usage() {
+	echo "Usage: ${0##*/} [-h] TARGET [BOARD]" >&2
+}
+
+print_help() {
+	print_usage
+	cat >&2 <<-EOF
+		  Use this script to checkout turris-build repository relative to build on
+		  repo.turris.cz. It not only checkouts exact commit in turris-build
+		  repository, but it also modifies feeds.conf file so exactly same feeds are
+		  used as they were used to build files on server.
+
+		TARGET argument is required and is target publishing branch (Do not mix
+		  this with git branch of same name). You can found all available targets
+		  on https://repo.turris.cz.
+		BOARD is optional argument used to specify different board. In default
+		  'omnia' is used. This is available because sometimes versions for boards
+		  are not in sync and can differ. It should be in most cases safe to use
+		  default but if you encounter problems you can try setting appropriate
+		  board.
+
+		Options:
+		  -h, --help  Print this help text
+	EOF
+}
+
+target=""
+board=""
+
+declare -A long2short=( ["help"]="h" )
+dashdash="n"
+while [ "$OPTIND" -le "$#" ]; do
+	argument="${!OPTIND}"
+	if [ "$dashdash" = "n" ] && getopts ":h" opt; then
+		if [ "${argument:0:2}" = "--" ]; then
+			longopt="${argument:2}"
+			if [ -v long2short["$longopt"] ]; then
+				opt="${long2short[$longopt]}"
+			else
+				OPTARG="$argument"
+			fi
+		fi
+		case "$opt" in
+			h)
+				print_help
+				exit 0
+				;;
+			\?)
+				echo "Illegal option: $OPTARG"
+				exit 1
+				;;
+		esac
+		continue
+	elif [ "$argument" == "--" -a "$dashdash" = "n" ]; then
+		dashdash="y"
+		continue
+	elif [ -z "$target" ]; then
+		target="$argument"
+	elif [ -z "$board" ]; then
+		board="$argument"
+	else
+		echo "Unexpected argument: $argument"
+		exit 1
+	fi
+	((OPTIND = OPTIND + 1))
+done
+
+[ -z "$target" ] && {
+	print_usage
+	exit 1
+}
+[ -z "$board" ] && board="omnia"
+
+[ "$(git rev-parse --is-inside-work-tree 2>/dev/null)" != "true" ] && \
+	die "Current working directory is not in git repository"
+
+toplevel="$(git rev-parse --show-toplevel)"
+
+hashes="$(curl --silent --location "https://repo.turris.cz/$target/$board/packages/git-hash" | \
+	awk 'NR > 1 && $1 == "*" { print $2 $3 }')"
+
+get_hash() {
+	awk -F ':' -vname="$1" '$1 == name { print $2 }' <<<"$hashes"
+}
+
+# Initially checkout turris-build
+git checkout "$(get_hash turris-build)"
+
+# And then modify feeds so we are modifying appropriate version
+while read -r tp name url vars; do
+	[ "$tp" = "#" ] && fullname="$name" || fullname="feeds/$name"
+	githash="$(get_hash "$fullname")"
+	if [ -n "$githash" ]; then
+		# Remove current branch/hash and append hash to URL
+		url="${url%%[;^]*}^$githash"
+	fi
+	if [ -n "$githash" ]; then
+		# Remove current branch/hash and append hash to URL
+		url="${url%%[;^]*}^$githash"
+	fi
+	echo "$tp" "$name" "$url" "$vars"
+done < "$toplevel/feeds.conf" > feeds.conf.new
+mv feeds.conf.new feeds.conf