hw/core/loader: implement address translation in uimage loader

Such address translation is needed when load address recorded in uImage
is a virtual address. When the actual load address is requested, return
untranslated address: user that needs the translated address can always
apply translation function to it and those that need it untranslated
don't need to do the inverse translation.

Add translation function pointer and its parameter to uimage_load
prototype. Update all existing users.

No user-visible functional changes.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>