postmarketos-base: post-install script will not update configs when updating postmarketos-base

This function allows overwriting configs from Alpine, unless they were modified by the user. The problem is, that when updating postmarketos-base, we might need to change the config yet again. But then apk audit will report that the file was modified, and so it does not get replaced. Let's get creative 😃

https://gitlab.postmarketos.org/postmarketos/pmaports/blob/85586ffbed39a0919e915fb65644f9a3b5267e46/main/postmarketos-base/postmarketos-base.post-install#L25

write_unless_modified() {
	# Redirect the stdin to the file unless it has been modified
	# $1: file path
	if ! apk audit "$(dirname "$1")" | grep -q "${1:1}"; then
		cat > "$1"
	fi
}