x86: zimage: Allow dumping the image from outside the module

At present it is possible to dump an image within the zimage command, but
it is also useful to be able to dump it from elsewhere, for example in a
loader that has special handling for the different zimage stages.

Export this feature as a new function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>