Meta: Factorize path resolution

This patch adds the `find_executable()` function that will hopefully
find executables in a distro-agnostic way and that is (hopefully as
well) easily upgradable.

The function uses some bash functionalities. So, we now require bash
for each script that includes `.shell_include.sh`.

Meta/build-image-extlinux.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 set -e