Fail if the cpio command is missing (v3)

Fail if the cpio command is missing (v3)

This one is rebased against today's git master, with an extra line for cpio.xz
-Martin
>From 388638cf72e6224fba4b96cda8db26fbc43bb20d Mon Sep 17 00:00:00 2001
From: Martin Panter <vadmium à gmail.com>
Date: Sun, 16 Jan 2011 04:38:03 +0000
Subject: [PATCH] IMAGE_CMD: Fail if the cpio command is missing

Previously my cpio.gz image was an empty gzip file because the missing cpio
command was ignored and gzip was blindly run with no input.

Signed-off-by: Martin Panter <vadmium+floss@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>