[OE-core] image.bbclass: Fix a couple innocuous typoes, should cause no functional change

[Richard Purdie]

On Thu, 2012-07-26 at 16:55 -0400, Robert P. J. Day wrote:
> "error_promt" -> "error_prompt" changed in both places
> "subfolers" -> "subfolders" since it's not referenced anyway
> 
> Signed-off-by: Robert P. J. Day <rpjday at crashcourse.ca>


Merged to master, thanks.

Richard