bitbake: hob2: remove the hard-coded images map

bitbake: hob2: remove the hard-coded images map

[Yocto #2795]

When a new image type added, the hob will crash because the new type is
not in the hard-coded image dictionary.

For most of the image types, they are same with the image file's
extension name. So use variable "IMAGE_EXTENSION_difftype" to map the
image type which is diff with the image file extension name, such as
type "live". And the variable(s) will be set in image_types.bbclass.

(Bitbake rev: e7c84f056af9c613920d5adcd078a011e0387193)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>