treewide: do not use IMG_PREFIX in Image/Prepare

treewide: do not use IMG_PREFIX in Image/Prepare

IMG_PREFIX can be modified in ImageBuilder by passing EXTRA_IMAGE_NAME
on command line, but Image/Prepare is not run in ImageBuilder. This
causes missing files when IMG_PREFIX is used for target file names in
Image/Prepare, then as source file names in Image/BuildKernel or
Image/Build.

Fix this by using a fixed output file name in Image/Prepare, and copy to
the expected file name in Image/BuildKernel instead, which is run by
ImageBuilder.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>