code cleanup for INITRD variable handing

* Remove an unnecessary check
* Instead of ignoring, report the errors

(From OE-Core rev: 7afc6df6f0d0bbe0c5cb8ec021d430d6d9714941)

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/bootimg.bbclass

@@ -75,6 +75,8 @@ populate() {
 		do
 			if [ -s "${fs}" ]; then
 				cat ${fs} >> ${DEST}/initrd
+			else
+				bbfatal "${fs} is invalid. initrd image creation failed."
 			fi
 		done
 		chmod 0644 ${DEST}/initrd