If the variable name is not specified then don't confuse the error message
by starting off with "Failure expanding variable None...".
(Bitbake rev: 9cb16f3c73751e7cf6d495586a6193f06eb97b1f)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>