cooker: Overwrite IMAGE_BASENAME to default in custom image

This solves a problem of custom images which inherit a base
image with IMAGE_BASENAME overwritten in their recipe by a
different value than its default one: ${PN}.

The value of IMAGE_BASE causes a crash when hob will try to
create symbolic links to the resulting images from the deploy
directory, because it will look for names similar to
<original_recipe_name>-edited-timestamp-machine.rootfs.*
which might be different from the actual resulting image.

The solution is to simply overwrite IMAGE_BASENAME in the
custom recipe to the default value in the case IMAGE_BASENAME
is found in the base recipe.

Some recipes which were affected by this problem are those
from meta-fsl-demos (e.g.: fsl-image-test).

[YOCTO #6017]

Signed-off-by: Marius Avram <marius.avram@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>